Refactor Launch

[4kangjc]

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What happened + What you expected to happen

在不改变函数调用形式的情况下，重构Launch，增强可读性，美观

// before
struct Launch {
    struct Prompt {};
    struct Schedule {};
    struct Current {};
};
// after
enum class Launch {
    Prompt,
    Schedule,
    Current,
};

// before
template <class T, class F,
          typename std::enable_if<std::is_same<T, Launch::Prompt>::value,
                                  T>::type* = nullptr>
inline Uthread async(F&& f, Executor* ex) {
    ...
}

template <class T, class F,
          typename std::enable_if<std::is_same<T, Launch::Schedule>::value,
                                  T>::type* = nullptr>
inline void async(F&& f, Executor* ex) {
    ...
}

// after
template <Launch T, class F>
inline std::enable_if_t<T == Launch::Prompt, Uthread> async(F&& f, Executor* ex) {
    ...
}

template <Launch T, class F>
inline std::enable_if_t<T == Launch::Schedule> async(F&& f, Executor* ex) {
   ...
}

Reproduction way

Anything else

Are you willing to submit a PR?

• Yes I am willing to submit a PR!