String tools.

Set:
 * TST-based set (ternary search tree)
 * Collapsed-TST-based set
Map:
 * TST-based map
 * TRIE map (Chris Okasaki map bootstraping implementation)
 * Radix-tree map (not really radix tree as each node store the whole key; but use critical bit for lookup)
Diff (string edition):
 * Diff: compute optimal alignment (no memory optimization; see app/fdiff example)