stream: impl Extend for StreamMap (#4272)

## 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`.

tokio-stream/src/stream_map.rs

@@ -585,6 +585,15 @@ where
     }
 }
 
+impl<K, V> Extend<(K, V)> for StreamMap<K, V> {
+    fn extend<T>(&mut self, iter: T)
+    where
+        T: IntoIterator<Item = (K, V)>,
+    {
+        self.entries.extend(iter);
+    }
+}
+
 mod rand {
     use std::cell::Cell;