Better support in StructArrays

Functions like `map` work on GPU arrays directly, but not on StructArrays-of-GPUArrays:
```julia
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 https://github.com/JuliaArrays/StructArrays.jl/issues/300

Start by examining the GPUArrays behavior for map and the StructArrays.jl issue referenced in the report. Reproduce the Julia example with StructArrays containing MtlArrays, then determine what support is needed so mapping over the struct array avoids scalar indexing and matches mapping over its GPU array fields.

Written by the indexing model from the issue text.