v0.2.2

Full Changelog: v0.2.1...v0.2.2

• Documentation improvements
• Add pending() and Pending
• Add empty() and Empty
• Add once() and Once
• Add poll_fn() and PollFn
• Add from_fn() and FromFn
• Add repeat_with() and RepeatWith
• Add once_with() and OnceWith
• Add repeat() and Repeat
• Add ready() and Ready