0.3 backports

[taiki-e]

Backports:

• Use FuturesOrdered in join_all (Use futuresordered in join_all #2412)
• New poll_immediate functions to immediately return from a poll (New poll_immediate functions to immediately return from a poll #2452)
• Add stream_select macro (Stream select macro #2262)
• Implement Default trait for OptionFuture (Implement Default trait for OptionFuture #2471)
• Prefer doc(no_inline) to doc(hidden) for reexports (Prefer doc(no_inline) to doc(hidden) for reexports #2479)
• Remove remaining cfg(target_has_atomic) (Remove remaining cfg(target_has_atomic) #2487)
• Add Peekable::{peek_mut, poll_peek_mut} (Add Peekable::{peek_mut, poll_peek_mut} #2488)
• Add BufReader::seek_relative (Add BufReader::{seek_relative, poll_seek_relative} #2489)