Supporting rules on functions with keyword arguments

[vchuravy]

Might be easier for 1.8 now that it's just kwfunc, but we need an isapplicable check for kwfuncs....
Maybe a generic handler for kwfuncs...

ChainRules has some hints https://github.com/JuliaDiff/ChainRulesCore.jl/blob/ed9a0073ff83cb3b1f4619303e41f4dd5d8c4825/src/rules.jl#L75-L79

cc: @vtjnash and @oxinabox

[vchuravy]

x-ref: JuliaDiff/ChainRules.jl#656

[vchuravy]

Thanks to @oxinabox for these hints

https://github.com/FluxML/Zygote.jl/blob/aea34ce61cae22f804ddf8a6d46e769694cdf0e3/src/compiler/interface2.jl#L11-L18
https://github.com/FluxML/Zygote.jl/blob/aea34ce61cae22f804ddf8a6d46e769694cdf0e3/src/compiler/interface2.jl#L11-L18

[vchuravy]

I wrote up a short snippet for checking if we have a rule.

function has_frule_kw_from_sig(@nospecialize(TT); world=Base.get_world_counter())
    ft, tt = _annotate_tt(TT)
    kwfunc = Core.kwfunc(EnzymeCore.EnzymeRules.forward)
    TT = Tuple{<:Any, typeof(forward), <:Annotation{ft}, Type{<:Annotation}, tt...}
    isapplicable(kwfunc, TT; world)
end   

But that needs testing. I think the big question is for how this works on the LLVM level. E.g maybe we first need to check that we can diff through kwargs.