update Rust edition -> 2021 #2980

Job	Run time
cargo test (ubuntu-latest)	2m 36s
cargo test (macos-latest)	6m 35s
cargo bench	44s
cargo +1.56 build (futures-{core, io, sink})	39s
cargo test (windows-latest)	4m 40s
cargo +stable build	39s
cargo +1.63 build (futures-{core, io, sink})	38s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 41s
cargo +beta build	36s
cargo hack check --feature-powerset	2m 20s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 13s
cargo +nightly build	35s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 39s
cargo minimal-versions build	23s
cargo miri test	13m 49s
cargo build --target thumbv7m-none-eabi	24s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 18s
cargo test -Z sanitizer=address	3m 27s
cargo build --target thumbv6m-none-eabi	24s
cargo +1.63 build	1m 6s
cargo test -Z sanitizer=memory	5m 28s
cargo test -Z sanitizer=thread	2m 33s
cargo clippy	0s
cargo doc	19s
cargo fmt	6s
	1h 7m 52s

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