ex 6 5: Implement undefine for hash table
ex 6 5: Implement undefine for hash table

Ex 6 5 asks me to 'write a function undef that will remove a name and
definition form the table maintained by lookup and install'. This is
just a matter of excising a node from a linked list, pretty
straightforward. I'm not sure about memory leaks though... I believe
this will become clearer in later chapters.

petosegan/krc-solutions