pub(crate) fn expect_class(self, msg: &str) -> Result<Module, RubyError> {
        //let self_ = self.clone();
        if self.is_class() {
            Ok(Module::new(self))
        } else {
            Err(VMError::wrong_type(msg, "Class", self))
        }
    }
Returns `ClassRef` if `self` is a Class.
When `self` is not a Class, returns `TypeError`.
ruruby/src/value/value.rs