add mapped_futures #2724

Job	Run time
cargo test (ubuntu-latest)	18s
cargo test (macos-latest)	20s
cargo bench	19s
cargo test (windows-latest)	39s
cargo +stable build	10s
cargo +1.36 build (futures-{core, io, sink})	46s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	1m 4s
cargo +beta build	18s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	39s
cargo +nightly build	14s
cargo hack check --feature-powerset	18s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	53s
cargo minimal-versions build	18s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	39s
cargo miri test	41s
cargo build --target thumbv7m-none-eabi	16s
cargo build --target thumbv6m-none-eabi	15s
cargo +1.56 build	51s
cargo clippy	14s
cargo doc	18s
cargo fmt	4s
cargo test -Z sanitizer=address	35s
cargo test -Z sanitizer=memory	36s
cargo test -Z sanitizer=thread	34s
	11m 19s

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