Constructs a region backed by a user-provided store. Assumes store is
aligned at minAlign. Also assumes the memory was allocated with ParentAllocator (if different from NullAllocator).
User-provided store backing up the region. store must be
aligned at minAlign (enforced with assert). If ParentAllocator is different from NullAllocator, memory is assumed to
have been allocated with ParentAllocator.
Constructs a region backed by a user-provided store. Assumes store is aligned at minAlign. Also assumes the memory was allocated with ParentAllocator (if different from NullAllocator).