Method deserialize_impl

tests/utils.h:318–322

  template <class T,
            typename std::enable_if<is_pair<T>::value>::type* = nullptr>
  T deserialize_impl() {
    auto first = deserialize_impl<typename T::first_type>();
    return std::make_pair(std::move(first),
                          deserialize_impl<typename T::second_type>());
  }

  template <class T, typename std::enable_if<
                         std::is_same<std::string, T>::value>::type* = nullptr>