"The method of killing a human child, one cannot even accurately say an entirely unborn human child, proscribed by this statute is so horrible that the most clinical description of it evokes a shudder of revulsion... the notion that the constitution of the United States, designed, among other things, 'to establish justice, insure domestic tranquillity, … and secure the blessings of liberty to ourselves and our posterity,' prohibits the states from simply banning this visibly brutal means of eliminating our half-born posterity is quite simply absurd."