Darts: Double-ARray Trie System

This is a C++ template library that implements Double-Array [Aoe
1989].  Currently only Japanese documentation is provided.

* [Aoe1989] Aoe, J. An Efficient Digital Search Algorithm by Using a
  Double-Array Structure. IEEE Transactions on Software
  Engineering. Vol. 15, 9 (Sep 1989). pp. 1066-1077.

* [Datrie] Theppitak Karoonboonyanan An Implementation of Double-Array
  Trie.

* [Word and Dictionary] Yuji Matsumoto et al. Word and Dictionary Iwanami
  Course Language Science Vol. 3 pp. 79-81.
