Implement the `UnwindSafe`/`RefUnwindSafe` traits on channels

[tomaka]

To my surprise, this trait isn't implemented on channels, even though channels are kind of a critical component to unwind safety.

See the code comment for an explanation of the reasoning.

I'm not 100% confident in what I'm doing, so feel free to summon unwind-safety experts, if there is any.

[taiki-e]

Thanks for the PR. I think we should make AtomicWaker UnwindSafe (#2211) first. (like tokio did)