void * coalesce(void * bp);

      param: bp - Points to a free block; not on the linked list (or
             tree) of free blocks.
      Description: Removes adjacent blocks from the free list (or
             tree) if either one or both are free. Merges block bp
             with these free adjacent blocks.

      Return: pointer to the merged block; also this block is NOT yet
             in the linked list or tree of free blocks.