This should be more robust during merges that happen simultaneously with
remote frees.
Previously, we would preserve the cache entry, but there is an edge case
where a slab of a lower index could be remotely freed and missed during
a merge.
- Simplify the malloc-based allocator
- Correct the type signatures on malloc/calloc/realloc
- Make `heap_alloc/heap_free/heap_resize` API consistent across platforms
- Add the test bench for the allocator
- Move old allocator code to the test bench
- Fix `heap_resize` usage in `os2/env_linux.odin` to fit new API
requiring `old_size`
