Support `?` operator in async stream (add async_try_stream)

[taiki-e]

Unlike futures-await, we cannot use ? operator in async_stream. It would be useful if we had more sugar for this.

#[async_try_stream(ok = String, error = io::Error)]
fn fetch_all(client: hyper::Client, urls: Vec<&'static str>) {
    for url in urls {
         yield fetch(client, url).await?;
    }
}

Related: rust-lang/futures-rs#1548 (comment)

[taiki-e]

Filed #15