add mapped_futures #2731

Job	Run time
cargo +1.36 build (futures-{core, io, sink})	50s
cargo bench	19s
cargo hack check --feature-powerset	15s
cargo minimal-versions build	23s
cargo +stable build	33s
cargo build --target thumbv7m-none-eabi	17s
cargo +beta build	21s
cargo test (ubuntu-latest)	20s
cargo build --target thumbv6m-none-eabi	17s
cargo +nightly build	22s
cargo +1.56 build	50s
cargo test (macos-latest)	19s
cargo test (windows-latest)	57s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	42s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	39s
cargo clippy	16s
cargo doc	18s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	39s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	39s
cargo fmt	4s
cargo miri test	35s
cargo test -Z sanitizer=address	45s
cargo test -Z sanitizer=memory	40s
cargo test -Z sanitizer=thread	38s
	11m 58s

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