pub(crate) fn superclass(&self) -> Option<Module> {
        let mut m = self.upper?;
        loop {
            if !m.is_included() {
                return Some(m);
            };
            m = m.upper?;
        }
    }
Get superclass of `self`.

If `self` has no superclass, return nil.
ruruby/src/value/class.rs