Add case-insensitive array

Summary: This class is case-insensitive but also case-preserving. The primary motivation for this class is to assist in handling PHP classes, functions and interfaces (which are case-insensitive).

Test Plan: Added test cases.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14624