Add `StreamExt::map_while` #2635

Job	Run time
cargo clippy	25s
cargo +stable build	40s
cargo +1.36 build (futures-{core, io, sink})	44s
cargo bench	46s
cargo +beta build	39s
cargo doc	19s
cargo +nightly build	45s
cargo hack check --feature-powerset	2m 24s
cargo fmt	14s
cargo miri test	9m 46s
cargo minimal-versions build	29s
cargo build --target thumbv7m-none-eabi	27s
cargo test -Z sanitizer=address	3m 21s
cargo build --target thumbv6m-none-eabi	23s
cargo test -Z sanitizer=memory	3m 45s
cargo test (ubuntu-latest)	3m 11s
cargo +1.56 build	1m 20s
cargo test -Z sanitizer=thread	2m 34s
cargo test (macos-latest)	6m 13s
cargo test (windows-latest)	5m 8s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 13s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 15s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 26s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 22s
	1h 2m 49s

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