Trapping of a carbocationic intermediate in the spontaneous hydrolysis reaction of 7.beta.,8.alpha.-dihydroxy-9.beta.,10.beta.-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene: mechanism of the spontaneous and general acid catalyzed hydrolysis reactions of bay-region benzo[a]pyrene 7,8-diol 9,10-epoxides