“Revealingly, the central function of the Constitution as law--the supreme law--was to impose limitations not on the behavior of ordinary citizens but on the federal government. The government, and those who ran it, were not placed outside the law, but expressly targeted by it. Indeed, the Bill of Rights is little more than a description of the lines that the most powerful political officials are barred from crossing, even if they have the power to do so and even when the majority of citizens might wish them to do so.”