optimize LruTimeCache::remove_expired_values, (#242)

by removing the double iteration

src/lru_time_cache.rs

@@ -88,17 +88,11 @@ impl<K: Clone + Eq + Hash, V> LruTimeCache<K, V> {
 
  /// Removes expired items from the cache.
  fn remove_expired_values(&mut self, now: Instant) {
- let mut expired_keys = vec![];
-
- for (key, (_, time)) in self.map.iter_mut() {
+ while let Some((_front, (_value, time))) = self.map.front() {
  if *time + self.ttl >= now {
  break;
  }
- expired_keys.push(key.clone());
- }
-
- for k in expired_keys {
- self.map.remove(&k);
+ self.map.pop_front();
  }
  }
 }