Special-case Complex?

[cossio]

using ZygoteStructArrays, Zygote, StructArrays
gradient(randn(2), randn(2)) do X, Y
    S = StructArray(Complex.(X, Y))
    sum(S.re) + sum(S.im)
end

fails, but

using ZygoteStructArrays, Zygote, StructArrays
struct Point
  x::Float64; y::Float64
end
gradient(randn(2), randn(2)) do X, Y
    S = StructArray(Point.(X, Y))
    sum(S.x) + sum(S.y)
end

works fine. The issue is that sensitivities for Complex in Zygote are not NamedTuple, see

FluxML/Zygote.jl#680