feat: add async study

__concurrent/async_study/examples/async2.rs

@@ -0,0 +1,23 @@
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+#[tokio::main]
+async fn main() {
+    invoke().await
+}
+
+struct FutureImpl {}
+
+impl Future for FutureImpl {
+    type Output = ();
+
+    fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
+        println!("Hello World!");
+        Poll::Ready(())
+    }
+}
+
+fn invoke() -> impl Future<Output=()> {
+    FutureImpl {}
+}