`Chain` forgets names under `fmap`

[mcabbott]

From FluxML/Optimisers.jl#49 (comment), this isn't great:

julia> fmap(identity, Chain(x=sin, y=cos))
Chain(sin, cos)

The culprit is this functor method, which could be extended to deal with this.

Flux.jl/src/layers/basic.jl

Line 44 in fe803a1

functor(::Type{<:Chain}, c) = c.layers, ls -> Chain(ls...)

Or we could delete it, and let Chain be more like Parallel:

julia> Flux.functor(Chain(x=sin, y=cos))
((x = sin, y = cos), Flux.var"#154#155"())

julia> Flux.functor(Parallel(vcat, (x=sin, y=cos)))
((connection = vcat, layers = (x = sin, y = cos)), Flux.var"#178#179"())

Then we would want delete the inner constructor for Chain, so that it accepts a tuple, Chain((sin, cos)) == Chain(sin, cos). That would line up with #1809 allowing Chain([sin, cos]) as a type-unstable variant.