4.25: feature release

This release:

• adds the Async\Poll, Async\Semaphore, and Async\Condition classes, previously in hsl-experimental; these are best avoided where possible - e.g. one-at-a-time queues can be built by waiting on the previous stored awaitable instead of using these classes.
• adds Math\is_nan() and the Math\NAN constant
• adds Dict\shuffle()
• separates type parameters for C\contains and C\contains_key;
• supports and requires HHVM 4.25 and above