Implement phutil_is_utf8_with_only_bmp_characters() without segfaulting
Summary: See comments. The regexp based implementation segfaults unpreventably on small inputs. Do it nice and slow in PHP instead.
Test Plan: Ran unit tests.
Reviewers: btrahan, arice
Reviewed By: arice
CC: chad, arice, aran
Differential Revision: https://secure.phabricator.com/D8312