"It was this dimension of personal liberty that Roe sought to protect, and its holding invoked the reasoning and the tradition of the precedents we have discussed, granting protection to substantive liberties of the person. Roe was, of course, an extension of those cases and, as the decision itself indicated, the separate States could act in some degree to further their own legitimate interests in protecting pre-natal life. The extent to which the legislatures of the States might act to outweigh the interests of the woman in choosing to terminate her pregnancy was a subject of debate both in Roe itself and in decisions following it."