internal/triegen: handle \xc0 and \xc1 correctly

\xc0 and \xc1 were seen as possible incomplete UTF-8 sequences,
whereas they are really always illegal. Fix the trie and updated
all code including some tests.

Change-Id: I5f9befc83ff0348d0ec4d5e1956d35387df00c03
Reviewed-on: https://go-review.googlesource.com/17495
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
8 files changed