message: remove needless clone
diff: @@ -140,10 +140,9 @@ where
  DB: Database,
  {
  fn try_fetch(&self, db: &DB, key: &Q::Key) -> Result<Q::Value, CycleDetected> {
  - let slot = match self.slot(key) {
  - Some(s) => s.clone(),
  - None => panic!("no value set for {:?}({:?})", Q::default(), key),
  - };
  + let slot = self.slot(key).unwrap_or_else(|| {
  + panic!("no value set for {:?}({:?})", Q::default(), key)
  + });
  let StampedValue { value, changed_at } = slot.stamped_value.read().clone();
  