phutil_utf8_shorten => PhutilUTF8StringTruncator

Summary: Ref T3307. Very easy

Test Plan: this very diff!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T3307

Differential Revision: https://secure.phabricator.com/D10391