Add `merge_round_robin` for StreamExt #3073

Job	Run time
cargo test (ubuntu-latest)	1m 14s
cargo test (macos-latest)	1m 22s
cargo minimal-versions build	24s
cargo +1.36 build (futures-{core, io, sink})	1m 10s
cargo test (windows-latest)	1m 12s
cargo +stable build	32s
cargo +beta build	38s
cargo test (ubuntu-24.04-arm)	1m 19s
cargo build --target thumbv7m-none-eabi	25s
cargo +1.63 build	1m 7s
cargo +nightly build	34s
cargo build --target thumbv6m-none-eabi	26s
cargo test (ubuntu-24.04-arm, armv7-unknown-linux-gnueabihf)	1m 19s
cargo bench	54s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	1m 20s
cargo clippy	24s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	1m 20s
cargo doc	19s
cargo hack check --feature-powerset	1m 19s
cargo fmt	6s
cargo miri test	1m 19s
cargo test -Z sanitizer=address	1m 19s
cargo test -Z sanitizer=memory	1m 20s
cargo test -Z sanitizer=thread	1m 19s
	22m 41s

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