## Motivation
This allows `StreamMap` to be used with [`futures::stream::StreamExt::collect`][collect].
My use case is something like this:
```rust
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 { ... }
```
[collect]: https://docs.rs/futures/0.3.17/futures/stream/trait.StreamExt.html#method.collect
## Solution
Add `Extend` impl that delegates to the inner `Vec`.
This change removes all references to `Stream` from
within the `tokio` crate and moves them into a new
`tokio-stream` crate. Most types have had their
`impl Stream` removed as well in-favor of their
inherent methods.
Closes#2870
David Pedersen