Add benchmark of ui.label

This is to help evaluate the impact of
https://github.com/emilk/egui/pull/302

egui_demo_lib/benches/benchmark.rs

@@ -48,6 +48,22 @@ pub fn criterion_benchmark(c: &mut Criterion) {
         });
     }
 
+    {
+        let mut ctx = egui::CtxRef::default();
+        ctx.begin_frame(raw_input.clone());
+        let mut ui = egui::Ui::__test();
+        c.bench_function("label &str", |b| {
+            b.iter(|| {
+                ui.label("the quick brown fox jumps over the lazy dog");
+            })
+        });
+        c.bench_function("label format!", |b| {
+            b.iter(|| {
+                ui.label(format!("the quick brown fox jumps over the lazy dog"));
+            })
+        });
+    }
+
     {
         let pixels_per_point = 1.0;
         let wrap_width = 512.0;