“[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.'”