[Feature]: Unmap Unused Allocator Pages

Feature Overview

Catch use-after-free bugs in Patina by unmapping unused pool allocator pages.

Solution Overview

1. Remove the 4K block list.
2. When allocating a sub 4K block - if no free available, go to the next free block and split it in half to put it in the lower sized block list (i.e. if no 8-byte blocks, grab the next 16-byte block, and allocate the first half, and mark the second half free and put it in the 8-byte list). Repeat until all block lists have been checked.
3. If no block lists can satisfy allocation, get a new allocation from fallback allocator and split it into two free entries in the 2048 bucket, and rerun 2.
4. if allocation is > 4K, simply go straight to fallback allocator and allocate whatever number of pages. For simplicity, fallback allocator would always allocate in page-size granularity - for > 4K allocations, that means that the remaining space in the page is just unused.
5. when free-ing a block, put free signature and length into the BlockListNode, then check from start of containing page to see if you can construct a whole free page.
6. if you can construct a whole page, remove all constituent blocks from the various free lists (simplified by adding a prev pointer so you can just remove them without finding them in the list) and free the corresponding page back to the fallback_allocator.

Alternatives Considered

No response

Urgency

Medium

Are you going to implement the feature request?

I will implement the feature

Do you need maintainer feedback?

No maintainer feedback needed

Anything else?

No response