This is an implementation of a doubly-linked list where
the forward and backward links are XOR'd together into a
single link.  This yields a list which takes the same
amount of memory as a singly-linked list, however allows
both forward and backward traversal.  

The nature of the links make it so that iteration must
start at either the head or the tail. 

Outstanding issues:
1) On a push* or a pop* operation, all existing iterators
are (silently) invalidated, as the iterators' internal
member pointers could become stale.

Author: Christopher Cole