Function select_biased 

pub(super) fn select_biased<S, T>(left: S, right: T) -> SelectBiased<S, T>
where
    S: Stream,
    T: Stream<Item = S::Item>,

Combine two instances of Stream into one.

This function is similar to futures::stream::select, but differs in that it treats the two underlying streams asymmetrically. Specifically:

• Each result is labeled with Source::Left or Source::Right, depending on which of the two streams it came from.
• If both the “left” and the “right” stream are ready, we always prefer the left stream.
• We stop iterating over this stream when there are no more results on the left stream, regardless whether the right stream is exhausted or not.

Future plans

This might need a better name, especially if we use it anywhere else.

If we do expose this function, we might want to split up the ways in which it differs from select.