Make RE2::Set canonicalize DFA states.
Make RE2::Set canonicalize DFA states.

Change-Id: Ib397b8fb7af447e62fbe8a30f75e3da8322e44a6
Reviewed-on: https://code-review.googlesource.com/c/re2/+/42570
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2