Class iterator

less_slow.cpp:3750–3759

    }

    struct iterator {
        std::coroutine_handle<promise_type> handle_;

        iterator &operator++() noexcept {
            handle_.resume();
            return *this;
        }
        bool operator!=(iterator const &) const noexcept { return !handle_.done(); }
        integer_type const &operator*() const noexcept { return handle_.promise().value_; }
    };

    iterator begin() noexcept {
        handle_.resume();