`@functor` for parametric constructors

Suppose I have a type
```
using Functors
struct Foo{T1,T2}
x::T2
Foo{T1}(x::T2) where {T1,T2} = new{T1,T2}(x)
end
```
Then using
```
@functor Foo
```
will not manage to capture the `T1` type:
```
x = Foo{Real}(2.0)
y, re = Functors.functor(x)
re(y)
ERROR: MethodError: no method matching Foo(::Float64)
Stacktrace:
[1] (::var"#5#6")(y::NamedTuple{(:x,), Tuple{Float64}})
@ Main ~/.julia/packages/Functors/qBIlC/src/functor.jl:23
[2] top-level scope
@ REPL[14]:1
```
using
```
@functor Foo{T} where {T}
```
is not helping more

Reproduce the parametric Foo example with @functor and inspect functor.jl around line 23, where the reconstruction call fails. Compare the behavior of @functor Foo and @functor Foo{T} where {T}; done means Functors.functor(x) retains the T1 parameter so re(y) succeeds.

Written by the indexing model from the issue text.