add mapped_futures #2725

Job	Run time
cargo bench	21s
cargo clippy	14s
cargo +1.36 build (futures-{core, io, sink})	50s
cargo +stable build	11s
cargo doc	17s
cargo +beta build	15s
cargo hack check --feature-powerset	17s
cargo fmt	4s
cargo +nightly build	15s
cargo minimal-versions build	21s
cargo miri test	44s
cargo build --target thumbv7m-none-eabi	17s
cargo test -Z sanitizer=address	35s
cargo build --target thumbv6m-none-eabi	15s
cargo test -Z sanitizer=memory	36s
cargo +1.56 build	52s
cargo test -Z sanitizer=thread	33s
cargo test (ubuntu-latest)	20s
cargo test (macos-latest)	23s
cargo test (windows-latest)	55s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	40s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	46s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	38s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	40s
	11m 19s

Provide feedback