Support shared futures on no_std #2951

Job	Run time
cargo +1.36 build (futures-{core, io, sink})	15s
cargo test (ubuntu-latest)	2m 48s
cargo test (macos-latest)	6m 24s
cargo test (windows-latest)	4m 38s
cargo bench	53s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 30s
cargo clippy	16s
cargo +stable build	31s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 17s
cargo +beta build	38s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 14s
cargo hack check --feature-powerset	41s
cargo miri test	13m 48s
cargo +nightly build	33s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 30s
cargo +1.56 build	9s
cargo test -Z sanitizer=address	3m 29s
cargo build --target thumbv7m-none-eabi	28s
cargo test -Z sanitizer=memory	5m 24s
cargo doc	17s
cargo test -Z sanitizer=thread	2m 27s
cargo build --target thumbv6m-none-eabi	25s
cargo fmt	8s
cargo minimal-versions build	23s
	1h 4m 6s

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