Fix Sync impl of FuturesUnordered #2332

Job	Run time
cargo bench	2m 28s
cargo clippy	39s
cargo +stable build	1m 22s
codegen	39s
cargo +beta build	1m 4s
cargo +1.36 build (futures-{core, io, sink})	3m 44s
cargo +nightly build	1m 4s
cargo doc	34s
cargo hack check --feature-powerset	3m 39s
cargo fmt	8s
cargo minimal-versions build	37s
cargo miri test	11m 7s
cargo build --target thumbv7m-none-eabi	38s
cargo test -Z sanitizer=address	5m 52s
cargo build --target thumbv6m-none-eabi	38s
cargo test (ubuntu-latest)	4m 21s
cargo test -Z sanitizer=memory	3m 46s
cargo +1.56 build	2m 58s
cargo test (macos-latest)	6m 20s
cargo test -Z sanitizer=thread	2m 53s
cargo test (windows-latest)	8m 24s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	5m 6s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	10m 17s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	8m 43s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	5m 15s
	1h 32m 16s

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