Upgrade to Rust 1.17.

- Remove some uses of 'static in const and static globals that are no
  longer needed.
- Use the new struct initialization shorthand.

src/cton-util.rs

@@ -17,7 +17,7 @@ mod cat;
 mod print_cfg;
 mod rsfilecheck;
 
-const USAGE: &'static str = "
+const USAGE: &str = "
 Cretonne code generator utility
 
 Usage: