Previously, a wide slab could be freed during this phase which could
trigger the superpage itself to be freed, resulting in iteration across
potentially invalid memory if the superpage was returned to the host and
not placed in the orphanage.
`heap_free_wide_slab` has had its responsibilities reduced to just
freeing the wide slab, and `heap_free_superpage_if_empty_and_unused` has
been added to better clarify what is happening where.
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`
