Add OptionalFuture helper #2721

Job	Run time
cargo +1.36 build (futures-{core, io, sink})	45s
cargo bench	19s
cargo clippy	13s
cargo +stable build	49s
cargo doc	17s
cargo +beta build	23s
cargo hack check --feature-powerset	15s
cargo +nightly build	22s
cargo fmt	7s
cargo minimal-versions build	17s
cargo miri test	43s
cargo build --target thumbv7m-none-eabi	17s
cargo test (ubuntu-latest)	17s
cargo test -Z sanitizer=address	34s
cargo build --target thumbv6m-none-eabi	17s
cargo test (macos-latest)	20s
cargo test -Z sanitizer=memory	38s
cargo +1.56 build	1m 18s
cargo test (windows-latest)	45s
cargo test -Z sanitizer=thread	37s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	40s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	45s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	38s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	35s
	12m 11s

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