Add "assert_same_keys()" to "libphutil/"

Summary:
This makes an implementation a little cleaner in an upcoming workboards diff.

This is like array_keys($x) === array_keys($y), but we get a slightly more useful error message, and we don't care about order.

Test Plan: Added and ran unit tests.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20268