add join for ThreadPool without destroying the threads (#2579)

* add join for ThreadPool without destroying the threads
* Make the main Thread block waiting for the worker threads to finish instead of buzy looping and do proper initialization and freeing of all variables.
* Updated test to use `atomic_store` and  take into account the maximum queue size of the threadpool.
* - Add `ThreadPool` join function to wait for all threads to finish in the pool without destroying the threads.
- Return of Thread/Mutex/CondVar `destroy()` is now "@maydiscard" and should be ignored. It will return void in 0.8.0.
- Return of Mutex `unlock()` and `lock()` is now "@maydiscard" and should be ignored. They will return void in 0.8.0.
- Return of ConditionVariable `signal()` `broadcast()` and `wait()` are now "@maydiscard". They will return void in 0.8.0.
- Return of Thread `detatch()` is now "@maydiscard". It will return void in 0.8.0.
- Buffered/UnbufferedChannel, and both ThreadPools have `@maydiscard` on a set of functions. They will retunr void in 0.8.0.
- Pthread bindings correctly return Errno instead of CInt.
- Return of Thread `join()` is now "@maydiscard".

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Co-authored-by: Christoffer Lerno <christoffer@aegik.com>

lib/std/experimental/FrameScheduler.c3

@@ -43,7 +43,7 @@ macro void FrameScheduler.@destroy(&self; @destruct(Event e))
 	self.events.free();
 	self.pending_events.free();
 	self.delayed_events.free();
-	(void)self.mtx.destroy();
+	self.mtx.destroy();
 }
 
 fn void FrameScheduler.queue_delayed_event(&self, Event event, Duration delay)