Speed up utf8_strlen()

Summary:

mb_strlen()            3.1 us
count(phutil_utf8v()) 28.3 us
strlen(utf8_decode())  1.7 us

Test Plan:

phutil_utf8_strlen('čeština');

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2280