"While it has engendered disapproval, it has not been unworkable. An entire generation has come of age free to assume Roe's concept of liberty in defining the capacity of women to act in society, and to make reproductive decisions; no erosion of principle going to liberty or personal autonomy has left Roe's central holding a doctrinal remnant; Roe portends no developments at odds with other precedent for the analysis of personal liberty; and no changes of fact have rendered viability more or less appropriate as the point at which the balance of interests tips. Within the bounds of normal stare decisis analysis, then, and subject to the considerations on which it customarily turns, the stronger argument is for affirming Roe's central holding, with whatever degree of personal reluctance any of us may have, not for overruling it."