“[T]here is no convincing reason why a patient who is acting completely on her own initiative, whether she be rich or poor, should be denied physician-assisted suicide because her decision turns, in whole or in part, on economic factors. If there remains a valid objection to economic motivations for physician-assisted suicide, it must stem from the concern that the patient is not acting completely on her own initiative but under pressure from others, from health care providers and managed care organizations, or from their families.”