sync: use Event for waitgroup on windows

vlib/sync/waiter_windows.c.v

@@ -0,0 +1,35 @@
+// Copyright (c) 2019 Alexander Medvednikov. All rights reserved.
+// Use of this source code is governed by an MIT license
+// that can be found in the LICENSE file.
+module sync
+
+// We cannot simply use Mutex.lock() in the waitgroup.wait() method
+// as it will not block since it's used in the same thread.
+// docs:
+// Any thread with a handle to a mutex object can use one of the
+// wait functions to request ownership of the mutex object.
+// If the mutex object is owned by another thread, the wait
+// function blocks the requesting thread until the owning thread
+// releases the mutex object using the ReleaseMutex function.
+// Source:
+// https://docs.microsoft.com/en-us/windows/win32/sync/mutex-objects
+
+[ref_only]
+struct Waiter{
+mut:
+	event MHANDLE
+}
+
+pub fn (mut w Waiter) wait() {
+	C.WaitForSingleObject(w.event, C.INFINITE) // infinite wait
+}
+
+pub fn (mut w Waiter) stop() {
+	C.SetEvent(w.event)
+}
+pub fn new_waiter() &Waiter {
+	unsafe {
+		sm := &Waiter{event: MHANDLE(C.CreateEvent(0, false, true, 0))}
+		return sm
+	}
+}