Add force_send method to channel Sender

[bergkvist]

Add a force_push method to the Sender part of a channel.

In particular, this is useful for bounded channels of non-zero size.

Motivation:

• https://github.com/crossbeam-rs/crossbeam/issues/400

Design goals:

• It is fine to lose messages, but messages should never appear in a different order from which they were sent
• For bounded (non-zero-sized) channels, force_send should never fail (except when the receiver has been dropped)
• Should not be blocking

use crossbeam_channel::{bounded, ForceSendError};

let (s, r) = bounded(3);

assert_eq!(s.force_send(0), Ok(None));
assert_eq!(s.force_send(1), Ok(None));
assert_eq!(s.force_send(2), Ok(None));
assert_eq!(s.force_send(3), Ok(Some(2)));

assert_eq!(r.recv(), Ok(0));

assert_eq!(s.force_send(4), Ok(None));

assert_eq!(r.recv(), Ok(1));
assert_eq!(r.recv(), Ok(3));

assert_eq!(s.force_send(5), Ok(None));
assert_eq!(s.force_send(6), Ok(None));
assert_eq!(s.force_send(7), Ok(Some(6)));
assert_eq!(s.force_send(8), Ok(Some(7)));

assert_eq!(r.recv(), Ok(4));
assert_eq!(r.recv(), Ok(5));
assert_eq!(r.recv(), Ok(8));

drop(r);

assert_eq!(s.force_send(9), Err(ForceSendError::Disconnected(9)));