sync: new_mutex() and new_waitgroup()

vlib/sync/waitgroup.v

@@ -4,12 +4,19 @@
 
 module sync
 
+//[init_with=new_waitgroup] // TODO: implement support for init_with struct attribute, and disallow WaitGroup{} from outside the sync.new_waitgroup() function.
 pub struct WaitGroup {
 mut:
 	mu Mutex
 	active int
 }
 
+pub fn new_waitgroup() WaitGroup {
+	mut w := WaitGroup{}
+	w.mu = sync.new_mutex()
+	return w
+}
+
 pub fn (wg mut WaitGroup) add(delta int) {
 	wg.mu.lock()
 	wg.active += delta