deallocate is defined if and only if at least one of the allocators
define deallocate. It works as follows. If primary.owns(b),
then the request is forwarded to primary.deallocate if it is defined,
or is a no-op otherwise. If primary does not own b, then the
request is forwarded to fallback.deallocate if it is defined, or is a
no-op otherwise.
deallocate is defined if and only if at least one of the allocators define deallocate. It works as follows. If primary.owns(b), then the request is forwarded to primary.deallocate if it is defined, or is a no-op otherwise. If primary does not own b, then the request is forwarded to fallback.deallocate if it is defined, or is a no-op otherwise.