define `rand_tangent` for `Dict`s

Currently it hits the fallback and errors:

julia> FiniteDifferences.rand_tangent(Dict("a"=>1))
ERROR: UndefRefError: access to undefined reference
Stacktrace:
  [1] getindex
    @ ./array.jl:801 [inlined]
  [2] _broadcast_getindex
    @ ./broadcast.jl:614 [inlined]
  [3] _getindex
    @ ./broadcast.jl:645 [inlined]
  [4] _getindex
    @ ./broadcast.jl:644 [inlined]
  [5] _broadcast_getindex
    @ ./broadcast.jl:620 [inlined]
  [6] getindex
    @ ./broadcast.jl:575 [inlined]
  [7] macro expansion
    @ ./broadcast.jl:984 [inlined]
  [8] macro expansion
    @ ./simdloop.jl:77 [inlined]
  [9] copyto!
    @ ./broadcast.jl:983 [inlined]
 [10] copyto!
    @ ./broadcast.jl:936 [inlined]
 [11] copy
    @ ./broadcast.jl:908 [inlined]
 [12] materialize
    @ ./broadcast.jl:883 [inlined]
 [13] rand_tangent(rng::Random._GLOBAL_RNG, x::Vector{String})
    @ FiniteDifferences ~/JuliaEnvs/test/alex/dev/FiniteDifferences/src/rand_tangent.jl:33
 [14] (::FiniteDifferences.var"#3#5"{Random._GLOBAL_RNG, Dict{String, Int64}})(field_name::Symbol)
    @ FiniteDifferences ~/JuliaEnvs/test/alex/dev/FiniteDifferences/src/rand_tangent.jl:52
 [15] map (repeats 2 times)
    @ ./tuple.jl:216 [inlined]
 [16] rand_tangent(rng::Random._GLOBAL_RNG, x::Dict{String, Int64})
    @ FiniteDifferences ~/JuliaEnvs/test/alex/dev/FiniteDifferences/src/rand_tangent.jl:51
 [17] rand_tangent(x::Dict{String, Int64})
    @ FiniteDifferences ~/JuliaEnvs/test/alex/dev/FiniteDifferences/src/rand_tangent.jl:8
 [18] top-level scope
    @ REPL[82]:1

we probably just need to write a dispatch of rand_tangent for Dicts, but it's kind of annoying because the keys can be different types and one can't map over dicts.

Start in src/rand_tangent.jl, especially the rand_tangent methods shown in the stack trace, and reproduce the Dict example from the issue. Define Dict support without relying on mapping over the dictionary; done means rand_tangent(Dict("a" => 1)) no longer reaches the fallback error and handles dictionaries whose keys have different types.