pub fn insert_new(&mut self, val: IdentId) -> Option<LvarId> {
        match self.table.get_lvarid(val) {
            Some(_) => None,
            None => {
                self.table.push(val);
                Some(LvarId::from(self.len() - 1))
            }
        }
    }
Add a new variable `val` to the `LvarCollector`.
Return None if `val` already exists.
ruruby-common/src/lvar_collector.rs