generic frule for constructors?

[oxinabox]

Like for #153
there is a generic frule for default constructors.
It relies on the arguments being the same as the fields so would have to detect that.

Its basically:

function frule(::Type{P}, args..., _, dargs...) where P
     y  = P(args...)
     dy = Composite{P}(; zip(fieldnames(P, dargs)...)
end

the more serious implementation might be:

function frule(::Type{P}, all_args...) where P
     nargs = fieldcount(P)
     length(all_args) == 2nargs + 1 || return nothing
     args = @inbounds all_args[1:nargs]
     all(typeof.(args) .== fieldtypes(P)) || return nothing
     dargs = @inbounds all_args[end-nargs : end]  # skip dself as constructors never functors
     y  = P(args...)
     dy = Composite{P}(; zip(fieldnames(P, dargs)...)
end

But this may well need to be written as a generated function if it doesn't constant fold good.

[oxinabox]

Related:

1. We need to have mutable composite types. MutableTangent ChainRulesCore.jl#105
2. We need a way to work out the differential type for a given primal type, so we can declare the differential arrays etc (this can default for Any) Function to find the Differential type for a given primal type ChainRulesCore.jl#106