(debtors who exempted a motor vehicle under state (Iowa) exemption statute are entitled to prove that vehicle is tool of the trade under 11 U.S.C. §522(f)(1)(B)(ii) in attempt to avoid nonpurchase-money security interest in the vehicle; bankruptcy court's determination that debtors could not exempt truck as tool of trade under state law and could not avoid lien under Bankruptcy Code lien avoidance provisions in error).