sync: add Mutex.destroy and RwMutex.destroy methods (#18351)

vlib/v/slow_tests/valgrind/sync.v

@@ -0,0 +1,20 @@
+import sync
+
+fn main() {
+	mut mutex := sync.new_mutex()
+	mutex.@lock()
+	mutex.unlock()
+	mutex.destroy()
+
+	mut rwmutex := sync.new_rwmutex()
+	rwmutex.@rlock()
+	rwmutex.unlock()
+	rwmutex.@lock()
+	rwmutex.unlock()
+	rwmutex.destroy()
+
+	mut sem := sync.new_semaphore()
+	sem.post()
+	sem.wait()
+	sem.destroy()
+}