Implement an iterator over the tree.

src/renderer/breadcrumb_tree.rs

@@ -37,3 +37,15 @@ impl<'a, C: IntoContextElement> BreadcrumbTree for BreadcrumbTreeNode<'a, C> {
         self.parent
     }
 }
+
+struct BreadcrumbTreeIterator<'a>(Option<&'a dyn BreadcrumbTree>);
+
+impl<'a> Iterator for BreadcrumbTreeIterator<'a> {
+    type Item = &'a dyn IntoContextElement;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let ret = self.0;
+        self.0 = self.0.map(|node| node.get_parent()).flatten();
+        ret.map(|node| node.get_ice())
+    }
+}