feat: add concurrent/waitgroup demo

__concurrent/waitgroup-demo/src/main.rs

@@ -0,0 +1,26 @@
+use std::time::Duration;
+
+use tokio::task::yield_now;
+use tokio::time;
+use tokio::time::Instant;
+use waitgroup::WaitGroup;
+
+#[tokio::main]
+async fn main() {
+    let wg = WaitGroup::new();
+    for i in 80..100 {
+        let w = wg.worker();
+        tokio::spawn(async move {
+            println!("Start work #{}", i);
+            time::sleep(Duration::from_millis(i * 100)).await;
+            println!("Finish work #{}", i);
+            drop(w);
+        });
+        yield_now().await;
+    }
+
+    let instant = Instant::now();
+    println!("Start waiting...");
+    wg.wait().await;
+    println!("Finish wait, elapsed: {}ms", instant.elapsed().as_millis());
+}