update Rust edition -> 2021 #2979

Job	Run time
cargo bench	48s
cargo clippy	0s
cargo +stable build	32s
cargo +1.56 build (futures-{core, io, sink})	34s
cargo +beta build	40s
cargo +1.63 build (futures-{core, io, sink})	39s
cargo doc	18s
cargo +nightly build	34s
cargo hack check --feature-powerset	2m 19s
cargo fmt	4s
cargo minimal-versions build	26s
cargo miri test	13m 51s
cargo build --target thumbv7m-none-eabi	25s
cargo build --target thumbv6m-none-eabi	23s
cargo test -Z sanitizer=address	3m 29s
cargo +1.63 build	1m 8s
cargo test (ubuntu-latest)	2m 30s
cargo test -Z sanitizer=memory	5m 26s
cargo test -Z sanitizer=thread	2m 27s
cargo test (macos-latest)	6m 54s
cargo test (windows-latest)	4m 47s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 24s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 17s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 36s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 26s
	1h 7m 57s

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