Make the suballocators !Sync

[marc0246]

Following up on #2316, the suballocators no longer need to be Sync like they used to because MemoryAlloc would hold a reference to them. Instead, this gives the user more freedom in how they synchronize the suballocator(s), if at all. In particular in the use case where one has thread-local suballocators there is no reason for this internal sync, but also it can lead to multiple layers of sync. GenericMemoryAllocator would use an RwLock to lock the pool, and then each block would use another Mutex internally for no reason in particular. (An argument for fine-grained locking can be made, however that isn't an argument for internal locking). The original intent when working on #1997 was to support both locking and lock-free suballocators, which sounded good in my head at the time but isn't practical at all when you think about it. I invite you to read the comments regarding this that are now gone (because the whole algorithm was simplified a bunch thanks to this) and facepalm in unison with me.

Changelog:

### Breaking changes
Changes to memory allocation:
- `FreeListAllocator`, `BuddyAllocator` and `BumpAllocator` are no longer `Sync`.