Micro-optimize idx()

Summary:
Do we call this function often enough to justify some speed tricks?

Tested in Zend PHP 5.4 (median of 3 runs):

idx($array, 'present'); // 36% faster
idx($array, 'missing'); // 23% faster
idx($array, 'missing', false); // 21% slower

Test Plan: New test.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D2672