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