Better support in StructArrays

[aplavin]

Functions like map work on GPU arrays directly, but not on StructArrays-of-GPUArrays:

using Metal

A = StructArray(a=MtlArray(rand(Float32, 10^3)), b=MtlArray(rand(Int8, 10^3)))

map(exp, A.a)  # works

map(x -> exp(x.a), A) # throws "Scalar indexing is disallowed" because `map` falls back to iteration

Would be great to make it work! How do you think could be implemented?
I'm not very experienced with GPU arrays and their inner workings.

Ref JuliaArrays/StructArrays.jl#300