fn consume_ident(&mut self) -> Result<Option<IdentId>, ParseErr> {
        match self.peek()?.kind {
            TokenKind::Ident(s) => {
                self.get()?;
                Ok(Some(self.get_ident_id(&s)))
            }
            _ => Ok(None),
        }
    }
If the next token is Ident, consume and return Some(it).
If not, return None.
ruruby-parse/src/parser.rs