Boundary tags, circular ddl-list, malloc, and free. Also mm_checkheap.

Implemented boundary tags so there's now a footer with the header.

Implemented a circular double linked list allowing the head pointer to
traverse bi-directionally and traverse where left off at.

Reimplemented malloc to use the list, which means that extend heap was
also changed to help and related functions.

Changed mm_checkheap to also go through the free list and verify nodes
are valid.