feat: add async exercises

solutions/21_async/async2.rs

@@ -0,0 +1,43 @@
+use tokio::task::JoinSet;
+
+// A MultiWorker can work with the power of 5 normal workers,
+// allowing us to create 5 new numbers at once!
+struct MultiWorker;
+
+impl MultiWorker {
+    async fn start_work(&self) -> JoinSet<i32> {
+        let mut set = JoinSet::new();
+
+        for i in 30..35 {
+            // TODO: `set.spawn` accepts an async function that will return the number
+            //       we want. Implement this function as a closure!
+            set.spawn(async move { i });
+        }
+
+        set
+    }
+}
+
+async fn run_multi_worker() -> Vec<i32> {
+    let tasks = MultiWorker.start_work().await;
+
+    // TODO: We have a bunch of tasks, how do we run them to completion
+    //       to get at the i32s they create?
+    tasks.join_all().await
+}
+
+fn main() {
+    // Feel free to experiment here. You may need to make some adjustments
+    // to this function, though.
+}
+
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn test_if_it_works() {
+        let mut numbers = run_multi_worker().await;
+        numbers.sort(); // in case tasks run out-of-order
+        assert_eq!(numbers, vec![30, 31, 32, 33, 34]);
+    }
+}