FuturesOrdered: handle index overflow correctly

[taiki-e]

See #2664

@conradludgate explained how to fix this, but it does not seem trivial.

(By the way, what I had in mind when I mentioned this issue in #2664 was to make the index 64-bit to mitigate the risk of overflow.)

[taiki-e]

push_back does not appear to decrement the index when futures are popped, so the code that uses push_back is probably more likely to be affected by this issue.

let mut set = FuturesOrdered::new();
for _ in 0..3 {
  set.push_back(std::future::ready(1));
  set.next().await;
  dbg!(set.len());
  dbg!(set.next_incoming_index);
  dbg!(set.next_outgoing_index);
}
/* result:
[futures-util/src/stream/futures_ordered.rs:254] set.len() = 0
[futures-util/src/stream/futures_ordered.rs:255] set.next_incoming_index = 1
[futures-util/src/stream/futures_ordered.rs:256] set.next_outgoing_index = 1
[futures-util/src/stream/futures_ordered.rs:254] set.len() = 0
[futures-util/src/stream/futures_ordered.rs:255] set.next_incoming_index = 2
[futures-util/src/stream/futures_ordered.rs:256] set.next_outgoing_index = 2
[futures-util/src/stream/futures_ordered.rs:254] set.len() = 0
[futures-util/src/stream/futures_ordered.rs:255] set.next_incoming_index = 3
[futures-util/src/stream/futures_ordered.rs:256] set.next_outgoing_index = 3
*/

let mut set = FuturesOrdered::new();
for _ in 0..3 {
  set.push_front(std::future::ready(1));
  set.next().await;
  dbg!(set.len());
  dbg!(set.next_incoming_index);
  dbg!(set.next_outgoing_index);
}
/* result:
[futures-util/src/stream/futures_ordered.rs:254] set.len() = 0
[futures-util/src/stream/futures_ordered.rs:255] set.next_incoming_index = 0
[futures-util/src/stream/futures_ordered.rs:256] set.next_outgoing_index = 0
[futures-util/src/stream/futures_ordered.rs:254] set.len() = 0
[futures-util/src/stream/futures_ordered.rs:255] set.next_incoming_index = 0
[futures-util/src/stream/futures_ordered.rs:256] set.next_outgoing_index = 0
[futures-util/src/stream/futures_ordered.rs:254] set.len() = 0
[futures-util/src/stream/futures_ordered.rs:255] set.next_incoming_index = 0
[futures-util/src/stream/futures_ordered.rs:256] set.next_outgoing_index = 0
*/