FuturesOrdered: Use 64-bit index #2410

Job	Run time
cargo +1.36 build (futures-{core, io, sink})	37s
cargo build --target thumbv7m-none-eabi	28s
cargo +stable build	43s
cargo build --target thumbv6m-none-eabi	29s
cargo +1.56 build	1m 7s
cargo +beta build	46s
cargo test (ubuntu-latest)	2m 47s
cargo +nightly build	44s
cargo bench	45s
cargo test (macos-latest)	6m 44s
codegen	25s
cargo test (windows-latest)	4m 14s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 30s
cargo hack check --feature-powerset	2m 45s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 25s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	7m 14s
cargo minimal-versions build	28s
cargo fmt	12s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 8s
cargo miri test	7m 34s
cargo clippy	27s
cargo test -Z sanitizer=address	2m 49s
cargo test -Z sanitizer=memory	3m 33s
cargo test -Z sanitizer=thread	2m 15s
	1h 0m 9s

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