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