[REF] observer: refactor observer using proxy

closes #253

tools/benchmarks/owl-master/app.js

@@ -107,12 +107,11 @@ class App extends owl.Component {
 
   updateSomeMessages() {
     this.benchmark("update every 10th", () => {
-      const setState = owl.core.Observer.set;
       const messages = this.state.messages;
       for (let i = 0; i < messages.length; i += 10) {
         const msg = Object.assign({}, messages[i]);
         msg.author += "!!!";
-        setState(messages, i, msg);
+        messages[i] = msg;
       }
     });
   }