add the method to access the last item in an array without chocking

Summary: Add method last() which is very similar to head()

Test Plan:

• this has been used in Facebook code like head().
• used it in D1344

Reviewers: epriestley, nh

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1343