Implemented phutil_utf8_combined().
Summary: A function which splits a given UTF-8 string into an array of characters without splitting the combined characters.
Test Plan: Unit tests.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin, AnhNhan
Maniphest Tasks: T2379
Differential Revision: https://secure.phabricator.com/D6011