Add constants.

tests/chars.rs

@@ -11,12 +11,12 @@ fn test_chars() -> Result<(), EvalAltResult> {
     {
         assert_eq!(engine.eval::<char>(r#"let x="hello"; x[2]"#)?, 'l');
         assert_eq!(
-            engine.eval::<String>(r#"let x="hello"; x[2]='$'; x"#)?,
+            engine.eval::<String>(r#"let y="hello"; y[2]='$'; y"#)?,
             "he$lo".to_string()
         );
     }
 
-    assert!(engine.eval::<char>("'\\uhello'").is_err());
+    assert!(engine.eval::<char>(r"'\uhello'").is_err());
     assert!(engine.eval::<char>("''").is_err());
 
     Ok(())