Implement pure version of todos example 🎉

The `Widget` trait in `iced_pure` needed to change a bit to make the
implementation of `Element::map` possible.

Specifically, the `children` method has been split into `diff` and
`children_state`.

pure/src/widget/text.rs

@@ -21,8 +21,10 @@ where
         Box::new(())
     }
 
-    fn children(&self) -> &[Element<Message, Renderer>] {
-        &[]
+    fn diff(&self, _tree: &mut Tree) {}
+
+    fn children_state(&self) -> Vec<Tree> {
+        Vec::new()
     }
 
     fn width(&self) -> Length {