message: Allow access to unordered_flat_maps's container
diff: @@ -153,6 +153,11 @@ public:
  return xs_.clear();
  }
  + /// Proves raw access to the underlying container.
  + vector_type& container() {
  + return xs_;
  + }
  +
  std::pair<iterator, bool> insert(value_type x) {
  return insert(end(), std::move(x));
  }
  