feat: `select!` macro for no-std environments #3000

Job	Run time
cargo +1.63 build	1m 16s
cargo test (ubuntu-latest)	2m 38s
cargo test (macos-latest)	5m 44s
cargo test (windows-latest)	4m 45s
cargo bench	43s
cargo +1.36 build (futures-{core, io, sink})	45s
cargo +stable build	38s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 21s
cargo +beta build	43s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 12s
cargo hack check --feature-powerset	2m 24s
cargo minimal-versions build	26s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 28s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 23s
cargo +nightly build	38s
cargo miri test	14m 30s
cargo build --target thumbv7m-none-eabi	27s
cargo build --target thumbv6m-none-eabi	23s
cargo test -Z sanitizer=address	3m 22s
cargo clippy	24s
cargo test -Z sanitizer=memory	5m 33s
cargo test -Z sanitizer=thread	2m 35s
cargo doc	18s
cargo fmt	10s
	1h 7m 46s

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