sync: use Event for waitgroup on windows

vlib/sync/waitgroup.v

@@ -15,13 +15,13 @@ struct WaitGroup {
 mut:
 	task_count       int // current task count
 	task_count_mutex &Mutex = &Mutex(0) // This mutex protects the task_count count in add()
-	wait_blocker     &Mutex = &Mutex(0) // This mutex blocks the wait() until released by add()
+	wait_blocker     &Waiter = &Waiter(0) // This blocks the wait() until released by add()
 }
 
 pub fn new_waitgroup() &WaitGroup {
 	return &WaitGroup{
 		task_count_mutex: new_mutex()
-		wait_blocker: new_mutex()
+		wait_blocker: new_waiter()
 	}
 }
 
@@ -36,14 +36,14 @@ pub fn (mut wg WaitGroup) add(delta int) {
 	}
 	// If task_count likely to leave zero, set wait() to block
 	if wg.task_count == 0 {
-		wg.wait_blocker.lock()
+		wg.wait_blocker.wait()
 	}
 	wg.task_count += delta
 	if wg.task_count < 0 {
 		panic('Negative number of jobs in waitgroup')
 	}
 	if wg.task_count == 0 { // if no more task_count tasks
-		wg.wait_blocker.unlock() // unblock wait()
+		wg.wait_blocker.stop() // unblock wait()
 	}
 }
 
@@ -54,6 +54,6 @@ pub fn (mut wg WaitGroup) done() {
 
 // wait blocks until all tasks are done (task count becomes zero)
 pub fn (mut wg WaitGroup) wait() {
-	wg.wait_blocker.lock() // blocks until task_count becomes 0
-	wg.wait_blocker.unlock() // allow other wait()s to unblock or reuse wait group
+	wg.wait_blocker.wait() // blocks until task_count becomes 0
+	wg.wait_blocker.stop() // allow other wait()s to unblock or reuse wait group
 }