update Rust edition -> 2021 #2978

Job	Run time
cargo test (ubuntu-latest)	2m 42s
cargo test (macos-latest)	5m 43s
cargo minimal-versions build	24s
cargo test (windows-latest)	4m 45s
cargo +stable build	42s
cargo +1.36 build (futures-{core, io, sink})	15s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 44s
cargo +1.41.0 build (futures-{core, io, sink})	18s
cargo +beta build	37s
cargo build --target thumbv7m-none-eabi	27s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 20s
cargo +1.56 build (futures-{core, io, sink})	35s
cargo +nightly build	33s
cargo build --target thumbv6m-none-eabi	25s
cargo +1.63 build (futures-{core, io, sink})	33s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 36s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 36s
cargo +1.56 build	1m 11s
cargo bench	41s
cargo +1.63 build	1m 10s
cargo clippy	15s
cargo doc	17s
cargo hack check --feature-powerset	2m 23s
cargo fmt	5s
cargo miri test	14m 56s
cargo test -Z sanitizer=address	3m 29s
cargo test -Z sanitizer=memory	5m 28s
cargo test -Z sanitizer=thread	2m 33s
	1h 10m 43s

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