Preserve @functor property order when reconstructing

[ToucheSir]

Fixes #68.

I originally hoped to have just the NamedTuple overload, but it turns out we're still documenting reconstructing from a plain tuple: https://github.com/FluxML/Functors.jl/blob/v0.4.4/src/Functors.jl#L49-L62.

PR Checklist

• Tests are added
  - [ ] Documentation, if applicable

[skyleaworlder]

As for plain tuple, it can be done using escfs like this (although assign_expr.args[1] might be a bit tricky):

      reconstruct(y) = begin 
          pos_val = Dict([(pos => val) for (pos, val) in enumerate(y)])
          pos_name = Dict([(pos => assign_expr.args[1]) for (pos, assign_expr) in enumerate($(escfs))])
          reconstruct(NamedTuple([(name, pos_val[pos]) for (pos, name) in pos_name]))
      end

julia> using Functors

julia> struct C
              a
              b
              c
              end

julia> @functor C (c, a)

julia> c = C(1,2,3)
C(1, 2, 3)

julia> cc, re = Functors.functor(c);

julia> map(float, cc)
(c = 3.0, a = 1.0)

julia> re(map(float, cc))
C(1.0, 2, 3.0)

julia> re((3.0, 1.0))
C(1.0, 2, 3.0)