Clean up and unify exercises

exercises/09_strings/strings4.rs

@@ -1,11 +1,7 @@
-// strings4.rs
-//
-// Ok, here are a bunch of values-- some are `String`s, some are `&str`s. Your
+// Ok, here are a bunch of values - some are `String`s, some are `&str`s. 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`
 // before the parentheses on each line. If you're right, it will compile!
-//
-// No hints this time!
 
 fn string_slice(arg: &str) {
     println!("{}", arg);