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::*;