Require a main function in all exercises

exercises/11_hashmaps/hashmaps2.rs

@@ -41,6 +41,10 @@ fn fruit_basket(basket: &mut HashMap<Fruit, u32>) {
     }
 }
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -79,7 +83,7 @@ mod tests {
         let count = basket.values().sum::<u32>();
         assert!(count > 11);
     }
-    
+
     #[test]
     fn all_fruit_types_in_basket() {
         let mut basket = get_fruit_basket();