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