“After the Sixteenth Amendment... the federal government... enacted a regime to tax incomes, gifts and estates... with the explicit promise that the basic means of sustaining life would not be taxed. The original tax regime applied only to the economic elite, to... "surplus" incomes. ...[I]ncome from capital was taxed more heavily ...in the belief that it was morally offensive to take more from money earned by the sweat of one's brow ...”