Method operator+

include/jwt/json/json.hpp:11293–11298

! @brief add to iterator @pre The iterator is initialized; i.e. `m_object != nullptr`. */

    @pre The iterator is initialized; i.e. `m_object != nullptr`.
    */
    iter_impl operator+(difference_type i) const
    {
        auto result = *this;
        result += i;
        return result;
    }

    /*!
    @brief  addition of distance and iterator