v3.0.0 - Unfair Lock

1. On iOS 10+ and macOS 10.12+ the locking mechanism for Promise is backed by os_unfair_lock bringing a significant performance improvement. On prior versions of iOS and macOS the locking mechanism falls back to a serial DispatchQueue
2. Expose a new Lock class that hides the implementation detail of the underlying lock, preferring os_unfair_lock when available, and falling back to a serial DispatchQueue when it's not.