Disallow overriding keywords.

src/engine.rs

@@ -187,6 +187,8 @@ pub const KEYWORD_FN_PTR_CURRY: &str = "curry";
 #[cfg(not(feature = "no_closure"))]
 pub const KEYWORD_IS_SHARED: &str = "is_shared";
 pub const KEYWORD_IS_DEF_VAR: &str = "is_def_var";
+#[cfg(not(feature = "no_function"))]
+pub const KEYWORD_IS_DEF_FN: &str = "is_def_fn";
 pub const KEYWORD_THIS: &str = "this";
 #[cfg(not(feature = "no_object"))]
 pub const FN_GET: &str = "get$";