“How could those who wrote the Constitution possibly understand its meaning better than those who had the experience of observing and participating in its operation? It is one thing to rail against the evils of politically unaccountable judges enlarging constitutional rights beyond the ideas and purposes of their adopters; another to explain why morally sustainable claims of equality be held captive to the extraordinary obstacles of Article V or subject to the partial and incomplete understandings of 1789 or 1868.”