fix signal dont triggered bug in collectAll & slot::emplace()

[poor-circle]

Why

slot::emplace()

When we call slot::emplace(), we check signal state before construct handler. however, when signal::emit() called, it will set signal state then set the construct handler.

A possible order:

1. Thread A: slot::emplace() check signal state, no triggered now.
2. Thread A: slot::emplace() construct signal handler.
3. Thread B: signal::emit() set signal state
4. Thread B: signal::emit() load signal handler and found it's nullptr, emit over
5. Thread A slot::emplace() set the signal handler by CAS successful. return true.

Now, slot::emplace() will return true and signal handler won't be executed. the signal is loss.

collectAny

collectAny should emit signal when the first handler resume, but now it may won't emit it if downCount was called in await_suspend

What is changing

slot::emplace()

slot::emplace() will construct signal handler before check signal state.

Because signal::emit() will always set state before load signal handler. so if signal::emit() load signal handler result is nullptr, slot::emplace() will always get the signal by check state.

collectAll and collectAny

Remove downCount call in await_suspend

Example

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@ChuanqiXu9 All ci passed

[ChuanqiXu9]

Now the coroutine may be suspended forever if the input was empty.

[ChuanqiXu9]

The variadic version has a static_assert for the inputs size.