pub fn insert_delegate_param(&mut self) -> Option<LvarId> {
        let lvar = self.insert_new(IdentId::get_id("..."))?;
        self.delegate_param = Some(lvar);
        Some(lvar)
    }
Add a delegate parameter `val` to the `LvarCollector`.
Return None if `val` already exists.
ruruby-common/src/lvar_collector.rs