Add functions for grabbing the first and last keys from an array.

Summary:
head_key($array);
and
last_key($array);

Test Plan:

<?php
$arr = array('Herp'=>1,'Derp',2);
echo head_key($arr);
echo last_key($arr);

Reviewers: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2161