Properly handle char types.

tests/chars.rs

@@ -6,6 +6,16 @@ fn test_chars() {
 
     assert_eq!(engine.eval::<char>("'y'"), Ok('y'));
     assert_eq!(engine.eval::<char>("'\\u2764'"), Ok('❤'));
+    assert_eq!(engine.eval::<char>(r#"let x="hello"; x[2]"#), Ok('l'));
+    assert_eq!(
+        engine.eval::<String>(r#"let x="hello"; x[2]='$'; x"#),
+        Ok("he$lo".into())
+    );
+
+    match engine.eval::<char>("'\\uhello'") {
+        Err(_) => (),
+        _ => assert!(false),
+    }
 
     match engine.eval::<char>("''") {
         Err(_) => (),