update Rust edition -> 2021 #2977

Job	Run time
cargo +1.56 build	1m 9s
cargo +1.36 build (futures-{core, io, sink})	15s
cargo +1.41.0 build (futures-{core, io, sink})	13s
cargo bench	43s
cargo +stable build	32s
cargo clippy	16s
cargo +beta build	38s
cargo hack check --feature-powerset	2m 20s
cargo +nightly build	32s
cargo doc	17s
cargo fmt	4s
cargo minimal-versions build	22s
cargo miri test	10m 49s
cargo build --target thumbv7m-none-eabi	25s
cargo test -Z sanitizer=address	3m 33s
cargo build --target thumbv6m-none-eabi	25s
cargo test -Z sanitizer=memory	5m 28s
cargo test (ubuntu-latest)	2m 44s
cargo test -Z sanitizer=thread	2m 30s
cargo test (macos-latest)	5m 46s
cargo test (windows-latest)	4m 37s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 27s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 0s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 27s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 33s
	1h 3m 5s

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