Add Suballocator::suballocations

[marc0246]

This allows the user to display debug information about individual memory blocks internal to the allocator. MemoryAllocator isn't integrated into this yet. This unfortunately required me to change Suballocator::allocate and Suballocator::deallocate to take &mut self, which makes it a bit more inconvenient to work with in many cases. But alas, the alternative would have been to keep the internal mutability but change it to a RefCell, and I'm not a fan of internal synchronization if it can be helped. It just means that the user will have to put the RefCell externally if needed.

Changelog:

### Breaking changes
Changes to memory allocation:
- `Suballocator::{allocate,deallocate}` now take `&mut self`.
- `Suballocator` has new required items `Suballocations` and `suballocations` for iterating over suballocations.