Require a main function in all exercises

exercises/05_vecs/vecs2.rs

@@ -26,6 +26,10 @@ fn vec_map(v: &Vec<i32>) -> Vec<i32> {
     }).collect()
 }
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;