Sync for allocators.

[rphmeier]

Sending allocators across thread boundaries is extremely useful. However, since most allocators are designed to have a reference to a parent allocator, Send for them requires that their parent is Sync to avoid race conditions in Drop. Allocators are basically required to have interior mutability, which rules out Sync for just about all of them except for HeapAllocator. Should there be a MutexAllocator/SyncAllocator type in the composable module that wraps an allocator in a mutex?