Support shared futures on no_std #2704

Job	Run time
cargo bench	21s
cargo +stable build	34s
cargo +1.36 build (futures-{core, io, sink})	40s
cargo +beta build	23s
cargo clippy	14s
cargo doc	19s
cargo +nightly build	23s
cargo hack check --feature-powerset	2m 5s
cargo fmt	13s
cargo minimal-versions build	25s
cargo miri test	37s
cargo build --target thumbv7m-none-eabi	17s
cargo build --target thumbv6m-none-eabi	18s
cargo test -Z sanitizer=address	51s
cargo test -Z sanitizer=memory	48s
cargo test (ubuntu-latest)	20s
cargo +1.56 build	45s
cargo test (macos-latest)	24s
cargo test (windows-latest)	44s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	46s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	1m 3s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	41s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	38s
cargo test -Z sanitizer=thread	38s
	14m 27s

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