update Rust edition -> 2021 #2976

Job	Run time
cargo +1.56 build	1m 9s
cargo +stable build	31s
cargo +beta build	38s
cargo +nightly build	35s
cargo +1.36 build (futures-{core, io, sink})	12s
cargo bench	44s
cargo clippy	14s
cargo doc	19s
cargo hack check --feature-powerset	2m 25s
cargo fmt	7s
cargo minimal-versions build	25s
cargo miri test	13m 47s
cargo build --target thumbv7m-none-eabi	25s
cargo test -Z sanitizer=address	3m 20s
cargo build --target thumbv6m-none-eabi	24s
cargo test (ubuntu-latest)	2m 40s
cargo test -Z sanitizer=memory	5m 30s
cargo test (macos-latest)	6m 45s
cargo test -Z sanitizer=thread	2m 43s
cargo test (windows-latest)	4m 55s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 31s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 13s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 34s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 24s
	1h 7m 30s

Provide feedback