Add Stream::timeout

Similar to the `future::timeout` free function we should add a `Stream::timeout` method. This method should be marked as "unstable".

## Example

```rust
// This example will never actually time out, but you get the idea.
let s = stream::repeat(1u8).take(100);
let s = s.timeout(Duration::from_secs(1));
while let Some(_) = s.next().await {
    dbg!();
}