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