[Allocator] Add Deallocate support to the AllocatorBase CRTP class,
along with templated overloads much like we have for Allocate. These
will facilitate switching the Deallocate interface of all the Allocator
classes to accept the size by pre-filling it from the type size where we
can do so. I plan to convert several uses to the template variants in
subsequent patches prior to adding the Size parameter.

No functionality changed, WIP.