Scalar indexing for `Base.hash`

Hello, I've recently noticed that GPUArrays don't define a `Base.hash` implementation and fallback to the default one. This requires one to `@allowscalar` which is slow and also means one has to differentiate between CPU and GPU arrays when calling hash.

MWE:

```julia
using GPUArrays, CUDA

A = cu(rand(1024, 1024))

hash(A) # errors

@allowscalar Base.hash(A) # slow
```

I'm not sure what a good implementation for GPU arrays would be. An inefficient GPU default could be:

```julia
Base.hash(arr::T, h) where T <: AbstractGPUArray = mapreduce(hash, hash, arr; init = hash(T, h))
```

That of course touches every element and works with `UInt64` values, but it would be faster than the normal default.

From what I can tell the default Base.hash for arrays is accessing O(log n) elements. I'm not sure how to neatly map such a pattern onto GPUs, if someone has any pointers I'd be happy to implement it