message: Fix iterator performance issue
diff: @@ -47,9 +47,9 @@ void init_ref_iterator(py::module& m, const std::string& it_name = typeid(T).nam
  })
  .def("__iter__",
  - [](T& v) -> T {
  - return std::begin(v);
  - }, py::return_value_policy::reference_internal)
  + [](const T& v) {
  + return py::make_iterator(std::begin(v), std::end(v));
  + }, py::keep_alive<0, 1>())
  .def("__next__",
  [] (T& v) -> typename T::reference {
  