feat: `select!` macro for no-std environments #2999

Job	Run time
cargo +1.63 build	1m 9s
cargo +stable build	38s
cargo +beta build	35s
cargo +1.36 build (futures-{core, io, sink})	45s
cargo bench	53s
cargo +nightly build	39s
cargo hack check --feature-powerset	1m 31s
cargo minimal-versions build	23s
cargo miri test	1m 32s
cargo build --target thumbv7m-none-eabi	25s
cargo test -Z sanitizer=address	1m 30s
cargo build --target thumbv6m-none-eabi	23s
cargo test (ubuntu-latest)	1m 18s
cargo test -Z sanitizer=memory	1m 29s
cargo test (macos-latest)	1m 30s
cargo test -Z sanitizer=thread	1m 29s
cargo clippy	22s
cargo test (windows-latest)	1m 17s
cargo doc	17s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	1m 30s
cargo fmt	12s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	1m 29s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	1m 29s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	1m 28s
	24m 13s

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