StreamExt::flat_map_unordered

[khuey]

(Inspired by the twitter thread at https://twitter.com/RReverser/status/1374544426622726145)

Flattening a stream of streams into a single stream provides an opportunity for concurrency if you don't care about the output order of the final stream. An efficient solution is difficult to realize with the built-in combinators.

• StreamExt::flat_map doesn't offer any concurrency
• stream::select_all requires collecting the stream of streams into an iterable.
• Using StreamAll directly can be done with StreamExt::collect, but it still requires collecting all streams from the stream of streams first before processing any of the output items.

What is really desired in this case is a combinator that introduces concurrency both within the set of streams yielded by the stream of streams and with the stream of streams itself. Something like (pseudocode)

    let stream_of_streams = ...;
    let accumulated_streams = SelectAll::new();
    loop {
        match select(stream_of_streams.next(), accumulated_streams.next()).await {
           StreamOfStreamsBranch(new_stream) => accumulated_streams.push(new_stream);
           AccumulatedStreamsBranch(item_to_pass_along) => break item_to_pass_along,
        }
    }

[taiki-e]

#1781 #2083