Support shared futures on no_std #3003

Job	Run time
cargo clippy	23s
cargo bench	48s
cargo +1.36 build (futures-{core, io, sink})	50s
cargo +stable build	41s
cargo +beta build	38s
cargo doc	19s
cargo hack check --feature-powerset	2m 32s
cargo fmt	20s
cargo +nightly build	37s
cargo minimal-versions build	22s
cargo miri test	13m 44s
cargo build --target thumbv7m-none-eabi	25s
cargo test -Z sanitizer=address	3m 30s
cargo build --target thumbv6m-none-eabi	24s
cargo test -Z sanitizer=memory	5m 51s
cargo test -Z sanitizer=thread	2m 34s
cargo test (ubuntu-latest)	2m 43s
cargo +1.63 build	1m 14s
cargo test (macos-latest)	5m 9s
cargo test (windows-latest)	4m 55s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 21s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 19s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 32s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 33s
	1h 7m 44s

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