Conjugated azoalkenes. Part 8. Reaction of some conjugated azoalkenes with activated nitriles. Synthesis of new pyrrolo[2,3-b]pyrroles and 1,2-diaminopyrroles. X-Ray molecular structure of diethyl 6-amino-1,6-bis-(t-butoxycarbonylamino)-3a-cyano-2,5-dimethyl-1,3a,6,6a-tetrahydropyrrolo-[2,3-b]pyrrol