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`.

Cargo.toml

@@ -91,6 +91,7 @@ members = [
     "examples/tour",
     "examples/url_handler",
     "examples/pure/counter",
+    "examples/pure/todos",
     "examples/websocket",
 ]