Utility for copying array to GPU

[gdalle]

Would this shortcut be welcome as a PR? I find it's one of the first thing I need when starting to play with KA, but perhaps I'm wrong.

function gpu(backend::Backend, x::Array)
    y = allocate(backend, eltype(x), size(x))
    return copyto!(y, x)
end

This is inspired by the Flux function of the same name.

[vchuravy]

The goal is to have adapt(backend, x) do this, but there might be a gap in documentation and tests.

[gdalle]

Right, looking again I see the related #501. Thanks