fn new_param(&mut self, id: IdentId, loc: Loc) -> Result<LvarId, ParseErr> {
        match self.context_mut().lvar.insert_new(id) {
            Some(lvar) => Ok(lvar),
            None => Err(error_unexpected(loc, "Duplicated argument name.")),
        }
    }
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