Reject nonminmal representations of UTF8 at the beginning of the 3-byte BMP range

Summary: Ref T1191. These byte ranges (\xE0\x80\x80 through \xE0\x9F\xBF) are alternate representations of characters with a different preferred minimal representation. MySQL and mbstring both reject them, and we should too.

Test Plan: Ran unit tests.

Reviewers: btrahan, arice

Reviewed By: arice

CC: chad, arice, aran

Maniphest Tasks: T1191

Differential Revision: https://secure.phabricator.com/D8313