Rename iced_virtual to iced_pure

`virtual` is a reserved keyword in Rust 😬

pure/src/widget/tree.rs

@@ -0,0 +1,58 @@
+use crate::widget::Element;
+
+use std::any::Any;
+use std::marker::PhantomData;
+
+pub struct Tree<Message, Renderer> {
+    pub state: Box<dyn Any>,
+    pub children: Vec<Tree<Message, Renderer>>,
+    types_: PhantomData<(Message, Renderer)>,
+}
+
+impl<Message, Renderer> Tree<Message, Renderer> {
+    pub fn new(element: &Element<Message, Renderer>) -> Self {
+        Self {
+            state: element.as_widget().state(),
+            children: element
+                .as_widget()
+                .children()
+                .iter()
+                .map(Self::new)
+                .collect(),
+            types_: PhantomData,
+        }
+    }
+
+    pub fn diff(
+        &mut self,
+        current: &Element<Message, Renderer>,
+        new: &Element<Message, Renderer>,
+    ) {
+        if current.as_widget().tag() == new.as_widget().tag() {
+            let current_children = current.as_widget().children();
+            let new_children = new.as_widget().children();
+
+            if current_children.len() > new_children.len() {
+                self.children.truncate(new_children.len());
+            }
+
+            for (child_state, (current, new)) in self
+                .children
+                .iter_mut()
+                .zip(current_children.iter().zip(new_children.iter()))
+            {
+                child_state.diff(current, new);
+            }
+
+            if current_children.len() < new_children.len() {
+                self.children.extend(
+                    new_children[current_children.len()..]
+                        .iter()
+                        .map(Self::new),
+                );
+            }
+        } else {
+            *self = Self::new(new);
+        }
+    }
+}