stream: impl Extend for StreamMap

[davidpdrsn]

Motivation

This allows StreamMap to be used with futures::stream::StreamExt::collect.

My use case is something like this:

let stream_map: StreamMap<_, _> = things
    .into_iter()
    .map(|thing| make_stream(thing)) // iterator of futures
    .collect::<FuturesUnordered<_>>() // stream of streams
    .collect::<StreamMap<_, _>>() // combine all the inner streams into one
    .await;

async fn make_stream(thing: Thing) -> impl Stream { ... }

Solution

Add Extend impl that delegates to the inner Vec.