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).
Bytes to allocate using ParentAllocator. This constructor is only
defined If ParentAllocator is different from NullAllocator. If
parent.allocate(n) returns null, the region will be initialized
as empty (correctly initialized but unable to allocate).
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).