Support shared futures on no_std #2995

Job	Run time
cargo +1.63 build	1m 11s
cargo +1.36 build (futures-{core, io, sink})	43s
cargo +stable build	39s
cargo test (ubuntu-latest)	2m 41s
cargo +beta build	40s
cargo test (macos-latest)	6m 21s
cargo bench	48s
cargo +nightly build	36s
cargo clippy	23s
cargo test (windows-latest)	4m 40s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 27s
cargo doc	17s
cargo hack check --feature-powerset	38s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 10s
cargo fmt	12s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	7m 28s
cargo minimal-versions build	23s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 35s
cargo miri test	14m 20s
cargo build --target thumbv7m-none-eabi	25s
cargo build --target thumbv6m-none-eabi	26s
cargo test -Z sanitizer=address	3m 28s
cargo test -Z sanitizer=memory	5m 44s
cargo test -Z sanitizer=thread	2m 32s
	1h 7m 47s

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