Fix UB in GenericMemoryAllocator::deallocate

[marc0246]

The code was UB for a two different reasons. First, the block pointer given out on allocation went through Box::deref_mut, which means that each new allocation made would invalidate pointers in previous allocations, in both SB and TB. Second, in SB, moving a Box asserts uniqueness, which happens when sorting the vec containing the boxes.

Sounds quite horrible, but thankfully, what's saving us is that deallocate is only ever called via dynamic dispatch unless you call it yourself. There's no way to take advantage of UB happening across dynamic dispatch calls.

Changelog:

### Bugs fixed
- Fixed UB in `GenericMemoryAllocator::deallocate` arising due to invalid pointer provenance given out on allocation.