Function add

include/covscript/impl/type_ext.hpp:34–37

	namespace operators {
		template <typename T>
		static inline var add(const T &lhs, const var &rhs)
		{
			throw lang_error("Type " + cs_impl::cxx_demangle(cs_impl::get_name_of_type<T>()) + " does not support + operator.");
		}

		template <typename T>
		static inline var sub(const T &lhs, const var &rhs)