add mapped_futures #2729

Job	Run time
cargo +stable build	8s
cargo clippy	15s
cargo +1.36 build (futures-{core, io, sink})	44s
cargo +beta build	17s
cargo bench	20s
cargo +nightly build	15s
cargo doc	16s
cargo hack check --feature-powerset	16s
cargo fmt	4s
cargo minimal-versions build	15s
cargo miri test	34s
cargo build --target thumbv7m-none-eabi	17s
cargo test -Z sanitizer=address	41s
cargo build --target thumbv6m-none-eabi	18s
cargo test (ubuntu-latest)	17s
cargo test -Z sanitizer=memory	36s
cargo +1.56 build	47s
cargo test (macos-latest)	20s
cargo test -Z sanitizer=thread	37s
cargo test (windows-latest)	51s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	41s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	36s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	39s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	34s
	10m 38s

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