fn new_kwrest_param(&mut self, id: IdentId, loc: Loc) -> Result<(), ParseErr> {
        if self.context_mut().lvar.insert_kwrest_param(id).is_none() {
            return Err(error_unexpected(loc, "Duplicated argument name."));
        }
        Ok(())
    }
Add the `id` as a new parameter in the current context.
If a parameter with the same name already exists, return error.
ruruby-parse/src/parser.rs