pub(crate) fn expect_mod_class(self) -> Result<Module, RubyError> {
        if self.if_mod_class().is_some() {
            Ok(Module::new(self))
        } else {
            Err(RubyError::typeerr(format!(
                "Must be Module or Class. (given:{:?})",
                self
            )))
        }
    }
Returns `ClassRef` if `self` is a Module / Class.
When `self` is not a Module, returns `TypeError`.
ruruby/src/value/value.rs