HAT-trie, an extremely efficient (space and time) modern variant of tries

 This is an ANSI C99 implementation of the HAT-trie data structure of Askitis
 and Sinha, an extremely efficient (space and time) modern variant of tries.
 .
 The version implemented here maps arrays of bytes to words (i.e., unsigned
 longs), which can be used to store counts, pointers, etc, or not used at all
 if you simply want to maintain a set of unique strings.