"The crux of any determination that a law unjustly discriminates against a group... is... that the law is part of a pattern that denies those subject to it a meaningful opportunity to realize their humanity. ...[S]uch an approach must look beyond process to identity and proclaim fundamental substantive rights—including substantive rights to participate on equal terms in the evolution of law and policy. ...[I]mportant aspects of constitutional law, including the determination of which groups deserve special protection, can be given content in no other way."