pub fn insert(&mut self, val: IdentId) -> LvarId {
        match self.table.get_lvarid(val) {
            Some(id) => id,
            None => {
                self.table.push(val);
                (self.len() - 1).into()
            }
        }
    }
Check whether `val` exists in `LvarCollector` or not, and return `LvarId` if exists.
If not, add new variable `val` to the `LvarCollector`.
ruruby-common/src/lvar_collector.rs