"[N]ot once in the history of the American Republic has this Court ever suggested the death penalty is categorically impermissible. The reason is obvious: It is impossible to hold unconstitutional that which the Constitution explicitly contemplates. The Fifth Amendment provides that "[n]o person shall be held to answer for a capital . . . crime, unless on a presentment or indictment of a Grand Jury," and that no person shall be "deprived of life . . . without due process of law.""