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`.

src/pure.rs

@@ -17,12 +17,15 @@
 //! [the original widgets]: crate::widget
 //! [`button::State`]: crate::widget::button::State
 //! [impure `Application`]: crate::Application
-pub use iced_pure::{Element as _, *};
+pub use iced_pure::{Element as _, Text as _, *};
 
 /// A generic, pure [`Widget`].
 pub type Element<'a, Message> =
     iced_pure::Element<'a, Message, crate::Renderer>;
 
+/// A pure text widget.
+pub type Text = iced_pure::Text<crate::Renderer>;
+
 mod application;
 mod sandbox;