feat: `select!` macro for no-std environments #2957

Job	Run time
cargo +1.36 build (futures-{core, io, sink})	33s
cargo +1.56 build	1m 9s
cargo bench	55s
cargo +stable build	30s
cargo hack check --feature-powerset	2m 16s
cargo +beta build	39s
cargo minimal-versions build	27s
cargo miri test	14m 16s
cargo +nightly build	33s
cargo build --target thumbv7m-none-eabi	16s
cargo test (ubuntu-latest)	2m 40s
cargo build --target thumbv6m-none-eabi	15s
cargo clippy	14s
cargo test (macos-latest)	5m 57s
cargo test (windows-latest)	4m 43s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 27s
cargo doc	19s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 36s
cargo fmt	5s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 33s
cargo test -Z sanitizer=address	3m 27s
cargo test -Z sanitizer=memory	5m 5s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	4m 26s
cargo test -Z sanitizer=thread	2m 37s
	1h 7m 58s

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