add mapped_futures #2727

Job	Run time
cargo clippy	16s
cargo +1.36 build (futures-{core, io, sink})	48s
cargo +stable build	10s
cargo bench	20s
cargo +beta build	15s
cargo doc	17s
cargo hack check --feature-powerset	14s
cargo +nightly build	16s
cargo fmt	4s
cargo minimal-versions build	17s
cargo miri test	34s
cargo build --target thumbv7m-none-eabi	17s
cargo test -Z sanitizer=address	36s
cargo build --target thumbv6m-none-eabi	16s
cargo test (ubuntu-latest)	17s
cargo test -Z sanitizer=memory	38s
cargo +1.56 build	49s
cargo test (macos-latest)	26s
cargo test -Z sanitizer=thread	36s
cargo test (windows-latest)	54s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	47s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	41s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	45s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	39s
	11m 12s

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