Require a main function in all exercises

exercises/03_if/if1.rs

@@ -10,6 +10,10 @@ pub fn bigger(a: i32, b: i32) -> i32 {
     // - additional variables
 }
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 // Don't mind this for now :)
 #[cfg(test)]
 mod tests {