Implement the `UnwindSafe`/`RefUnwindSafe` traits on channels #2266

Job	Run time
cargo +1.36 build (futures-{core, io, sink})	2m 15s
cargo test (ubuntu-latest)	4m 23s
cargo test (macos-latest)	5m 52s
cargo test (windows-latest)	6m 10s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	4m 13s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	8m 21s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	8m 54s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	5m 3s
codegen	32s
cargo bench	1m 13s
cargo hack check --feature-powerset	3m 38s
cargo +stable build	1m 4s
cargo +beta build	1m 17s
cargo +nightly build	1m 2s
cargo minimal-versions build	35s
cargo build --target thumbv7m-none-eabi	38s
cargo build --target thumbv6m-none-eabi	40s
cargo +1.56 build	1m 49s
cargo clippy	32s
cargo doc	28s
cargo fmt	11s
cargo miri test	9m 0s
cargo test -Z sanitizer=address	5m 12s
cargo test -Z sanitizer=memory	3m 19s
cargo test -Z sanitizer=thread	3m 47s
	1h 20m 8s

Provide feedback