feat: add async exercises

exercises/21_async/async1.rs

@@ -0,0 +1,44 @@
+// Our loyal worker works hard to create a new number.
+#[derive(Default)]
+struct Worker;
+
+struct NumberContainer {
+    number: i32,
+}
+
+impl Worker {
+    async fn work(&self) -> NumberContainer {
+        // Pretend this takes a while...
+        let new_number = 32;
+        NumberContainer { number: new_number }
+    }
+}
+
+impl NumberContainer {
+    async fn extract_number(&self) -> i32 {
+        // And this too...
+        self.number
+    }
+}
+
+// TODO: Fix the function signature!
+fn run_worker() -> i32 {
+    // TODO: Make our worker create a new number and return it.
+}
+
+fn main() {
+    // Feel free to experiment here. You may need to make some adjustments
+    // to this function, though.
+}
+
+mod tests {
+    use super::*;
+
+    // Don't worry about this attribute for now.
+    // If you want to know what this does, read the hint!
+    #[tokio::test]
+    async fn test_if_it_works() {
+        let number = run_worker().await;
+        assert_eq!(number, 32);
+    }
+}