"There are, we feel, two important interests that a state possesses and that if it so desires, it may seek to protect by legislation. The first is the state’s interest in preserving and protecting the health of the pregnant woman. The second is the state’s interest in protecting the potentiality of human life, irrespective of the moment when life actually begins. These interests are separate and distinct. Each grows in substantiality as the woman approaches term, and at some point during pregnancy each becomes “compelling.”"