Add phutil_utf8_codepoints(), phutil_utf8_console_strlen()

Summary: Reasonably approximate a console-aware display width function for arbitrary utf8 strings so I can make D3568 work correctly.

Test Plan: Unit tests.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1831

Differential Revision: https://secure.phabricator.com/D3569