message: imhamt: add contains_key()
diff: @@ -123,6 +123,10 @@ impl<H: Hasher + Default, K: Hash + Eq, V> Hamt<H, K, V> {
  }
  }
  }
  + /// Check if the key is contained into the HAMT
  + pub fn contains_key(&self, k: &K) -> bool {
  + self.lookup(k).map_or_else(|| false, |_| true)
  + }
  pub fn iter(&self) -> HamtIter<K, V> {
  HamtIter {
  stack: vec![self.root.iter()],
  