Implement array_fuse()

Summary: We've been bitten by the behavior of array_combine() on empty arrays in PHP < 5.4 several times recently. Implement an alternative for array_combine(x, x).

Test Plan: Unit tests.

Reviewers: btrahan, chad, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4657