Return impl IntoElement from RenderOnce::render

This makes it a bit more approachable to implement.

This required converting the state of Component to AnyElement, which
costs an allocation. I'm hoping this will be ok performance-wise now
that AnyElements use bump allocation, but I need to benchmark still.

crates/ui2/src/components/label/label.rs

@@ -40,9 +40,7 @@ impl LabelCommon for Label {
 }
 
 impl RenderOnce for Label {
-    type Output = LabelLike;
-
-    fn render(self, _cx: &mut WindowContext) -> Self::Output {
+    fn render(self, _cx: &mut WindowContext) -> impl IntoElement {
         self.base.child(self.label)
     }
 }