FutureAwaiter Support Cancellation

[poor-circle]

Why

FutureAwaiter should adapt signal cancellation.

What is changing

Add support of cancellation for FutureAwaiter.

Now co_await std::move(future) could be canceled by signal.

Example

template <typename Callback>
void sum(int a, int b, Callback&& callback) const {
    std::thread([callback = std::move(callback), a, b]() mutable {
        callback(a + b);
    }).detach();
}
async_simple::executors::SimpleExecutor ex1(2);
void test() {
    auto lazy = [&]() -> Lazy<> {
        Promise<int> pr;
        auto fut = pr.getFuture();
        sum(1, 1, [pr = std::move(pr)](int val) mutable {
            std::this_thread::sleep_for(std::chrono::seconds::max());
            pr.setValue(val);
        });
        async_simple::SignalType type = None;
        try {
            co_await std::move(fut);
        } catch (const async_simple::SignalException& e) {
            type = e.value();
        } catch (...) {
        }
        EXPECT_EQ(type, async_simple::Terminate);
    };
    syncAwait(collectAll<async_simple::Terminate>(
        lazy().via(&ex1),
        async_simple::coro::sleep(std::chrono::microseconds{10}).via(&ex1)));
}

[ChuanqiXu9]

LGTM.