volatile_memory: add VolatileArrayRef and &std::sync::atomic::Atomic* accessors

[bonzini]

Allow easily access to an array stored in a VolatileMemory object, removing
the need to do multiplications and pointer offsetting.

[alexandruag]

Hi Paolo,

I was just wondering, is there any fundamental reason why we should have both VolatileSlice and VolatileArrayRef? Right now, it seems VolatileSlice basically corresponds to a byte slice, whereas VolatileArrayRef behaves like a slice of some type T. Does VolatileArrayRef still make sense if we parametrize VolatileSlice<T>? We can implement the common functionality in VolatileSlice<T>, and move all methods that only make sense for a byte slice in an impl VolatileSlice<u8> { ... } or something along those lines.

[bonzini]

That's an interesting idea. I will take a look.

[bonzini]

The main obstacle here is that VolatileSlice has some extra methods to copy in and out of the slice, compared to VolatileArrayRef, and I don't think they make sense for VolatileArrayRef (better: they do make sense, but with a different type bound, they can be T: ByteValued for VolatileSlice but they should not be generic for VolatileArrayRef. However, I've added a From implementation to go from VolatileArrayRef<u8> to VolatileSlice, and a bunch of other improvements.