Constructs a region backed by a user-provided store. Assumes the memory was allocated with ParentAllocator (if different from `NullAllocator`).
User-provided store backing up the region. If ParentAllocator is different from `NullAllocator`, memory is assumed to have been allocated with ParentAllocator.
See Implementation
Constructs a region backed by a user-provided store. Assumes the memory was allocated with ParentAllocator (if different from `NullAllocator`).