Implement FusedStream for FuturesOrdered

[phil-opp]

This uses the same approach as the FusedStream implementation for FuturesUnordered.

Fixes #2192

cc @taiki-e

[taiki-e]

FuturesUnordered implements FusedStream with the same logic that this PR introduces, and is_terminated field is true only when in_progress_queue.poll_next_unpin() returns None, so I think self.in_progress_queue.is_terminated() && self.queued_outputs.is_empty() is enough for this.

[phil-opp]

Thanks for the suggestion, I pushed a new commit with it.

[phil-opp]

The clippy failure in src/mpsc/mod.rs seems unrelated to this PR.

[taiki-e]

Yes, that will be fixed in #2203

[taiki-e]

Thanks!