Add benchmark button to UI

The benchmark button is only enabled in release builds. If clicked, it
will trigger the benchmark suite.

examples/data.myc

@@ -1,5 +1,5 @@
 ;; Complex Data Structure Test
-;; Benchmark: 49.8us
+;; Benchmark: 50.2us
 ;; Output: {:input 10, :name "Fibonacci", :output 55}
 (do
   (def fib (fn [n]

examples/macro_hygiene.myc

@@ -1,3 +1,4 @@
+;; Benchmark: 300ns
 ;; Output: 5
 (do
 (def y 1)

src/main.rs

@@ -397,6 +397,18 @@ impl eframe::App for CompilerApp {
                 {
                     self.run_all_tests();
                 }
+
+                ui.separator();
+
+                let is_debug = cfg!(debug_assertions);
+                ui.add_enabled_ui(!is_debug, |ui| {
+                    let bench_btn = ui.button("⏱ Bench");
+                    if is_debug {
+                        bench_btn.on_hover_text("Benchmarks are only available in Release builds.");
+                    } else if bench_btn.clicked() {
+                        self.run_benchmarks(false);
+                    }
+                });
             });
             ui.add_space(2.0);
         });