fix some clippy lints

src/media_playlist.rs

@@ -383,6 +383,7 @@ impl MediaPlaylist {
 
     /// Computes the `Duration` of the [`MediaPlaylist`], by adding each segment
     /// duration together.
+    #[must_use]
     pub fn duration(&self) -> Duration {
         self.segments.values().map(|s| s.duration.duration()).sum()
     }

src/types/key_format_versions.rs

@@ -184,7 +184,7 @@ impl KeyFormatVersions {
     /// ```
     #[inline]
     #[must_use]
-    pub fn is_empty(&self) -> bool { self.len() == 0 }
+    pub const fn is_empty(&self) -> bool { self.len() == 0 }
 
     /// Removes the last element and returns it, or `None` if it is empty.
     ///