void * extend_heap(size_t nwords)

      param: nwords - number of 4 byte words
      Description: Allocates a new free block of size which is a
             multiple of 8 immediately after the last block. Merges
             this new block with the last block if that block is free.
             Rewrites an eiplog block after the new block.
      
      Return: pointer to the new free  block (NOT yet in the linked
              list or tree of free blocks)