Update to SQLite 3.8.0

This updates the unicodesn tokenizer with the latest code from today's
release of SQLite 3.8.0.  The only relevant change is a bug fix where
now all private code points are treated as identifier symbols (see
<https://www.sqlite.org/releaselog/3_8_0.html>; I couldn't find a bug
report about it).
Merge pull request #2 from chazomaticus/master

Update to SQLite 3.8.0