Provide implementations for Sink and Stream

I think it would be a great addition to also provide `Sink` and `Stream` implementations for `Duplex`.

```rust
impl<T, R: Stream<T>, W> Stream<T> for Duplex<R, W> {}

impl<T, R, W: Sink<T>> Sink<T> for Duplex<R, W> {}
```

I’d be happy to provide a PR.