This allows the pooling allocator to precisely control how each allocation should be dropped.
This is important to the pooling allocator, because it needs to know what arena, chunk, page, run, etc. is being freed, exactly.
Fundamental design issues remain, though.
Drops can end up being shared across instances with different memory allocations, and this means we can't currently attach the de-allocation information to the drop instance.
We also cannot use the AllocationControl instance for this because it has the same problem.
