“The Court granted certiorari in these cases to consider whether the death penalty is any longer a permissible form of punishment. It is the judgment of five Justices that the death penalty, as customarily prescribed and implemented in this country today, offends the constitutional prohibition against cruel and unusual punishments. The reasons for that judgment are stated in five separate opinions, expressing as many separate rationales. In my view, none of these opinions provides a constitutionally adequate foundation for the Court’s decision.”