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_input.rs

@@ -146,8 +146,10 @@ where
         Box::new(text_input::State::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 {
@@ -237,3 +239,16 @@ where
         text_input::mouse_interaction(layout, cursor_position)
     }
 }
+
+impl<'a, Message, Renderer> From<TextInput<'a, Message, Renderer>>
+    for Element<'a, Message, Renderer>
+where
+    Message: 'a + Clone,
+    Renderer: 'a + text::Renderer,
+{
+    fn from(
+        text_input: TextInput<'a, Message, Renderer>,
+    ) -> Element<'a, Message, Renderer> {
+        Element::new(text_input)
+    }
+}