Arena allocator

[schell]

These changes replace SlabAllocator with Arena.

craballoc is interesting in that it is essentially a bump-allocated arena that runs on top of a u32 slab. These changes refactor and rename SlabAllocator to Arena to reflect that fact, as well as provide a more concise API and some new features, mainly:

• round-trip synchronization of values
• opt out for CPU-cached values
• opt out for GPU synchronization

These changes also include rather rigorous proptest tests.

[LegNeato]

FWIW I have been looking at https://github.com/SFBdragon/talc for something similar though I have yet to really use it. One nice feature is on OOM you can call back to the host and request more memory if needed or aggressively reap.

[schell]

Thanks @LegNeato. That looks pretty similar. Currently craballoc doesn't handle OOM, as it just allocs more, but eventually it would run into the runtime's buffer maximum. That would be a hard error though.