feat: add async study

__concurrent/async_study/examples/async3.rs

@@ -0,0 +1,35 @@
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+#[tokio::main]
+async fn main() {
+    invoke().await
+}
+
+struct FutureImpl {
+    i: i32,
+}
+
+impl Future for FutureImpl {
+    type Output = ();
+    
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        if self.i == 0 {
+            let s = &mut *self;
+            s.i += 1;
+            println!("Hello World, pending");
+            cx.waker().wake_by_ref();
+            Poll::Pending
+        } else {
+            println!("Hello World, ready");
+            Poll::Ready(())
+        }
+    }
+}
+
+fn invoke() -> impl Future<Output=()> {
+    FutureImpl {
+        i: 0,
+    }
+}