Function assign_sum

include/chaiscript/dispatchkit/operators.hpp:84–87

      template<typename T>
        void assign_sum(Module& m)
        {
          m.add(chaiscript::fun([](T &lhs, const T&rhs)->T&{return lhs += rhs;}), "+=");
        }

      template<typename T>
        void prefix_decrement(Module& m)