4-oxospiro[benzopyran-2,4'-piperidines] as class III antiarrhythmic agents. Pharmacological studies on 3,4-dihydro-1'-[2-(benzofurazan-5-yl)-ethyl]-6-methanesulfonamidospiro[(2H)-1-benzopyran-2,4'-piperidin]-4-one (L-691,121)