message: Document VecMap panics
  Summary: Following diffs removes the panic.
diff: @@ -369,6 +369,7 @@ impl<K, V> VecMap<K, V> {
  Some(value)
  } else {
  let i = self.values.len();
  + // Panics here if current size is equal to `THRESHOLD`
  self.hashes[i] = key.hash();
  self.values.push((key.into_key(), value));
  None
  