Survey HAT-trie to be the next generation of DARTS

[MnO2]

It seems to be the fastest trie but more complicated to implement.

• https://github.com/Tessil/hat-trie
• https://tessil.github.io/2017/06/22/hat-trie.html
• http://crpit.scem.westernsydney.edu.au/confpapers/CRPITV62Askitis.pdf
• https://github.com/luikore/triez

[MnO2]

From a brief reading it seems that HAT-trie doesn't support commonPrefix, at least it is not provided in Tessil's hat-trie.

And cedar's double array trie implementation looks pretty good, it incorporates some latest techniques so that the insertion no longer slow, and the look-up speed comparable with HAT-trie

• http://www.tkl.iis.u-tokyo.ac.jp/~ynaga/papers/ynaga-coling2014.pdf
• http://www.tkl.iis.u-tokyo.ac.jp/~ynaga/cedar/

[messense]

Close this issue since #62 is merged.