Method try_pop

include/workspace/taskqueue.hpp:42–50

        q.emplace_front(std::move(v));
    }
    bool try_pop(T& tmp) {
        std::lock_guard<std::mutex> lock(tq_lok);
        if (!q.empty()) {
            tmp = std::move(q.front());
            q.pop_front();
            return true;
        }
        return false;
    }
    size_type length() {
        std::lock_guard<std::mutex> lock(tq_lok);
        return q.size();