Added filename to top of .rs files

strings/strings1.rs

@@ -1,3 +1,4 @@
+// strings1.rs
 // Make me compile without changing the function signature! Scroll down for hints :)
 
 fn main() {

strings/strings2.rs

@@ -1,3 +1,4 @@
+// strings2.rs
 // Make me compile without changing the function signature! Scroll down for hints :)
 
 fn main() {

strings/strings3.rs

@@ -1,3 +1,4 @@
+// strings3.rs
 // Ok, here are a bunch of values-- some are `Strings`, some are `&strs`. Your
 // task is to call one of these two functions on each value depending on what
 // you think each value is. That is, add either `string_slice` or `string`