feat: add async exercises

solutions/22_macros/macros1.rs

@@ -0,0 +1,10 @@
+macro_rules! my_macro {
+    () => {
+        println!("Check out my macro!");
+    };
+}
+
+fn main() {
+    my_macro!();
+    //      ^
+}

solutions/22_macros/macros2.rs

@@ -0,0 +1,10 @@
+// Moved the macro definition to be before its call.
+macro_rules! my_macro {
+    () => {
+        println!("Check out my macro!");
+    };
+}
+
+fn main() {
+    my_macro!();
+}

solutions/22_macros/macros3.rs

@@ -0,0 +1,13 @@
+// Added the attribute `macro_use` attribute.
+#[macro_use]
+mod macros {
+    macro_rules! my_macro {
+        () => {
+            println!("Check out my macro!");
+        };
+    }
+}
+
+fn main() {
+    my_macro!();
+}

solutions/22_macros/macros4.rs

@@ -0,0 +1,15 @@
+// Added semicolons to separate the macro arms.
+#[rustfmt::skip]
+macro_rules! my_macro {
+    () => {
+        println!("Check out my macro!");
+    };
+    ($val:expr) => {
+        println!("Look at this other macro: {}", $val);
+    };
+}
+
+fn main() {
+    my_macro!();
+    my_macro!(7777);
+}