Added filename to top of .rs files

standard_library_types/arc1.rs

@@ -1,3 +1,4 @@
+// arc1.rs
 // Make this code compile by filling in a value for `shared_numbers` where the
 // TODO comment is and creating an initial binding for `child_numbers`
 // somewhere. Try not to create any copies of the `numbers` Vec!

standard_library_types/iterator3.rs

@@ -1,3 +1,4 @@
+// iterator3.rs
 // This is a bigger exercise than most of the others! You can do it!
 // Here is your mission, should you choose to accept it:
 // 1. Complete the divide function to get the first four tests to pass

standard_library_types/iterators4.rs

@@ -1,3 +1,5 @@
+// iterators4.rs
+
 pub fn factorial(num: u64) -> u64 {
     // Complete this function to return factorial of num
     // Do not use: