Some refactoring of the rust generated code

Always use a Pin<Rc> for the component. (This is required to support repeater
within repeater as well anyway)
Do not use the context within the binding. We can get along by simply capturing
a weak pointer to the component

examples/rusttest2/src/main.rs

@@ -3,7 +3,7 @@
 sixtyfps::include_modules!();
 
 fn main() {
-    let mut app = Hello::default();
+    let app = Hello::new();
 
     app.plus_clicked.set_handler(|context, ()| {
         let app = context.get_component::<Hello>().unwrap();