refactor: do not get domains as often

src/main.rs

@@ -30,7 +30,7 @@ async fn complete_channel(mut channel: Channel, client: &Client) -> Result<Box<C
     let grouped: Vec<(Option<String>, Vec<rss::Item>)> = channel
         .items()
         .iter()
-        .sorted_by(|a, b| get_domain(a).cmp(&get_domain(b)))
+        .sorted_by_cached_key(|a| get_domain(a))
         .chunk_by(|&item| get_domain(item))
         .into_iter()
         .map(|(k, v)| (k, v.cloned().collect()))