StackOverflowError when using callable syntax

MWE:

```
julia> using AxisKeys, Distributions, KeyedDistributions

julia> keys = ([:a, :b, :c], );

julia> d = KeyedDistribution(MvNormal(rand(3)), keys)
KeyedMvNormal{ZeroMeanDiagNormal{Tuple{Base.OneTo{Int64}}}}(
d: ZeroMeanDiagNormal(
dim: 3
μ: Zeros(3)
Σ: [0.083005916265146 0.0 0.0; 0.0 0.6484896856503174 0.0; 0.0 0.0 0.04377962068806388]
)

keys: ([:a, :b, :c],)
)

julia> d(:a)
Internal error: encountered unexpected error in runtime:
StackOverflowError()
subtype_ccheck at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
var_gt at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
forall_exists_subtype at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)

subtype_ccheck at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
var_gt at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
forall_exists_equal at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
subtype at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
with_tvar at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
subtype at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
with_tvar at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
subtype at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
with_tvar at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
subtype at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
subtype_tuple_tail at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
subtype at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
with_tvar at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
subtype at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
with_tvar at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
subtype at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
with_tvar at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
subtype at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
with_tvar at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
subtype at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
forall_exists_subtype at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
jl_subtype_env at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
jl_type_intersection_env_s at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
ml_matches at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
jl_matching_methods at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
_methods_by_ftype at ./reflection.jl:867 [inlined]
#findall#216 at ./compiler/methodtable.jl:57 [inlined]
findall##kw at ./compiler/methodtable.jl:54
jfptr_findallYY.YY.kw_13388.clone_1 at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/sys.dylib (unknown line)
jl_apply_generic at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
#218 at ./compiler/methodtable.jl:67
get! at ./iddict.jl:163 [inlined]
#findall#217 at ./compiler/methodtable.jl:66 [inlined]
findall##kw at ./compiler/methodtable.jl:65
jfptr_findallYY.YY.kw_13383.clone_1 at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/sys.dylib (unknown line)
jl_apply_generic at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
abstract_call_gf_by_type at ./compiler/abstractinterpretation.jl:84
abstract_call_known at ./compiler/abstractinterpretation.jl:1033
abstract_call at ./compiler/abstractinterpretation.jl:1056
abstract_call at ./compiler/abstractinterpretation.jl:1040
abstract_eval_statement at ./compiler/abstractinterpretation.jl:1167
typeinf_local at ./compiler/abstractinterpretation.jl:1462
typeinf_nocycle at ./compiler/abstractinterpretation.jl:1520
_typeinf at ./compiler/typeinfer.jl:214
typeinf at ./compiler/typeinfer.jl:209
typeinf_edge at ./compiler/typeinfer.jl:806
abstract_call_method at ./compiler/abstractinterpretation.jl:490
abstract_call_gf_by_type at ./compiler/abstractinterpretation.jl:143
abstract_call_known at ./compiler/abstractinterpretation.jl:1033
abstract_call at ./compiler/abstractinterpretation.jl:1056
abstract_call at ./compiler/abstractinterpretation.jl:1040
abstract_eval_statement at ./compiler/abstractinterpretation.jl:1167
typeinf_local at ./compiler/abstractinterpretation.jl:1462
typeinf_nocycle at ./compiler/abstractinterpretation.jl:1520
_typeinf at ./compiler/typeinfer.jl:214
typeinf at ./compiler/typeinfer.jl:209
typeinf_edge at ./compiler/typeinfer.jl:806
abstract_call_method at ./compiler/abstractinterpretation.jl:490
abstract_call_gf_by_type at ./compiler/abstractinterpretation.jl:143
abstract_call_known at ./compiler/abstractinterpretation.jl:1033
abstract_call at ./compiler/abstractinterpretation.jl:1056
abstract_call at ./compiler/abstractinterpretation.jl:1040
abstract_eval_statement at ./compiler/abstractinterpretation.jl:1167
typeinf_local at ./compiler/abstractinterpretation.jl:1462
typeinf_nocycle at ./compiler/abstractinterpretation.jl:1520
_typeinf at ./compiler/typeinfer.jl:214
typeinf at ./compiler/typeinfer.jl:209
typeinf_edge at ./compiler/typeinfer.jl:806
abstract_call_method at ./compiler/abstractinterpretation.jl:490
abstract_call_gf_by_type at ./compiler/abstractinterpretation.jl:143
abstract_call_known at ./compiler/abstractinterpretation.jl:1033
abstract_call at ./compiler/abstractinterpretation.jl:1056
abstract_call at ./compiler/abstractinterpretation.jl:1040
abstract_eval_statement at ./compiler/abstractinterpretation.jl:1167
typeinf_local at ./compiler/abstractinterpretation.jl:1462
typeinf_nocycle at ./compiler/abstractinterpretation.jl:1520
_typeinf at ./compiler/typeinfer.jl:214
typeinf at ./compiler/typeinfer.jl:209
typeinf_ext at ./compiler/typeinfer.jl:892
typeinf_ext_toplevel at ./compiler/typeinfer.jl:925
typeinf_ext_toplevel at ./compiler/typeinfer.jl:921
jfptr_typeinf_ext_toplevel_13551.clone_1 at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/sys.dylib (unknown line)
jl_apply_generic at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
jl_type_infer at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
jl_generate_fptr at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
jl_compile_method_internal at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
jl_apply_generic at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
do_call at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
eval_body at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
jl_interpret_toplevel_thunk at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
top-level scope at REPL[26]:1
jl_toplevel_eval_flex at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
jl_toplevel_eval_flex at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
jl_toplevel_eval_flex at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
jl_toplevel_eval_in at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
eval at ./boot.jl:360 [inlined]
eval_user_input at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/REPL/src/REPL.jl:139
repl_backend_loop at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/REPL/src/REPL.jl:200
start_repl_backend at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/REPL/src/REPL.jl:185
#run_repl#42 at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/REPL/src/REPL.jl:317
run_repl at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/REPL/src/REPL.jl:305
jl_apply_generic at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
#874 at ./client.jl:387
jfptr_YY.874_23727.clone_1 at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/sys.dylib (unknown line)
jl_apply_generic at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
jl_f__call_latest at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
#invokelatest#2 at ./essentials.jl:708 [inlined]
invokelatest at ./essentials.jl:706 [inlined]
run_main_repl at ./client.jl:372
exec_options at ./client.jl:302
_start at ./client.jl:485
jfptr__start_37496.clone_1 at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/sys.dylib (unknown line)
jl_apply_generic at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
true_main at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
repl_entrypoint at /Applications/Julia-1.6.app/Contents/Resources/julia/lib/julia/libjulia-internal.1.6.dylib (unknown line)
KeyedDistribution{Univariate, Continuous, Normal{Float64}}(
d: Normal{Float64}(μ=0.0, σ=0.083005916265146)
keys: ([:a],)
)
```

Note that it still returns the correct result, but I have seen it cause total failure in downstream code.

I don't think this is covered by tests as they still pass locally without this error. It doesn't seem to be a problem for `FullNormal`. The code below is adapted from the "marginalising" testset:

```julia
using StableRNGs

X = rand(StableRNG(1234), 10, 3)
m = vec(mean(X; dims=1))
s = cov(X; dims=1)

d = KeyedDistribution(MvNormal(m, s), keys)

d(:a)

# output
KeyedDistribution{Univariate, Continuous, Normal{Float64}}(
d: Normal{Float64}(μ=0.5753194713591709, σ=0.0839957973357219)
keys: ([:a],)
)
```

Start with the KeyedDistribution callable syntax shown in the MWE and compare it with the existing “marginalising” testset; the issue names no source files. Reproduce the StackOverflowError on Julia 1.6 with the diagonal MvNormal case, then verify that d(:a) completes without the compiler failure and that the relevant tests cover it.

Written by the indexing model from the issue text.