Add trie simple implementation

Trie provides very effective performance for symbol tables with
string keys.

Search hit:  need to examine L(length of the key)
             characters for equality.
Search miss:
             o Could have mismatch on first character
	     o Typical case: examine only a few characters (sublinear)

Space:       R(alphabet size) null links at each leaf

Bottom line: fast search hit and fast search miss, but wastes space