Implement pure::Component in iced_lazy

pure/src/widget/button.rs

@@ -209,12 +209,12 @@ where
     }
 
     fn overlay<'b>(
-        &'b mut self,
+        &'b self,
         tree: &'b mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
     ) -> Option<overlay::Element<'b, Message, Renderer>> {
-        self.content.as_widget_mut().overlay(
+        self.content.as_widget().overlay(
             &mut tree.children[0],
             layout.children().next().unwrap(),
             renderer,