add mapped_futures #2730

Job	Run time
cargo test (ubuntu-latest)	18s
cargo test (macos-latest)	26s
cargo test (windows-latest)	52s
cargo bench	20s
cargo +1.36 build (futures-{core, io, sink})	46s
cargo +stable build	35s
cargo +beta build	22s
cargo +nightly build	21s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	47s
cargo minimal-versions build	16s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	38s
cargo build --target thumbv7m-none-eabi	16s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	40s
cargo build --target thumbv6m-none-eabi	15s
cargo +1.56 build	1m 53s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	39s
cargo clippy	15s
cargo doc	16s
cargo hack check --feature-powerset	15s
cargo fmt	5s
cargo miri test	43s
cargo test -Z sanitizer=address	35s
cargo test -Z sanitizer=memory	36s
cargo test -Z sanitizer=thread	37s
	12m 46s

Provide feedback