Require a main function in all exercises

exercises/07_structs/structs1.rs

@@ -14,6 +14,10 @@ struct ColorTupleStruct(/* TODO: Something goes here */);
 #[derive(Debug)]
 struct UnitLikeStruct;
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

exercises/07_structs/structs2.rs

@@ -28,6 +28,10 @@ fn create_order_template() -> Order {
     }
 }
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

exercises/07_structs/structs3.rs

@@ -38,6 +38,10 @@ impl Package {
     }
 }
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;