“America is the promised land, because each generation bequeathed to its children a promise, a promise that they might not come to enjoy but which they fully expected their offspring to fulfill. So the words 'all men are created equal' took a life of its own, ultimately destined to end slavery and enfranchise women. And the words 'equal protection' and 'due process' inevitably led to the end of the words 'separate but equal,' ensuring that the walls of segregation would crumble, whether at the lunch counter or at the voting booth.”