Implementation of a Incrementally Defragmenting Auto-Sizing Multi-Arena Allocator

[douira]

Refactors the buffer arena to consist of multiple large buffers that are shared by multiple regions with logic to incrementally defragment, allocate, and deallocate them and move regions between them as buffers fill up.

Remaining todos:

• Limit maximum buffer allocation size more aggressively, even with weird resource packs it's problematic to create too large buffers
• Add ability to reduce size of buffer when there's just one big buffer left over
• Scale amount of allowed defragmentation work with size of buffer and number of segments as it doesn't keep up otherwise

Closes #2683
Closes #2233