consider switching from hash table for large allocations to an arena-based red-black tree to satisfy range requests for malloc_object_size

[thestinger]

The current implementation will lead to only being able to provide size results for offsets into the initial page(s) of a large allocation, since with a hash table there isn't a way to find the closest previous match.

This would have a performance cost compared to the hash table for allocation, and would also make malloc_object_size significantly slower. It might make sense as a configuration option, or perhaps it's not worth doing at all.