Constructs a block allocator given a hunk of memory, or a desired capacity in bytes.

• If ParentAllocator is `NullAllocator`, only the constructor taking data is defined and the user is responsible for freeing data if desired.
• Otherwise, both constructors are defined. The data-based constructor assumes memory has been allocated with the parent allocator. The capacity-based constructor uses ParentAllocator to allocate an appropriate contiguous hunk of memory. Regardless of the constructor used, the destructor releases the memory by using ParentAllocator.deallocate.