"The right of a competent, terminally ill person to avoid excruciating pain and embrace a timely and dignified death bears the sanction of history and is implicit in the concept of ordered liberty. The exercise of this right is as central to personal autonomy and bodily integrity as rights safeguarded by this Court's decisions relating to marriage, family relationships, procreation, contraception, child rearing and the refusal or termination of life-saving medical treatment. In particular, this Court's recent decisions concerning the right to refuse medical treatment and the right to abortion instruct that a mentally competent, terminally ill person has a protected liberty interest in choosing to end intolerable suffering by bringing about his or her own death. A state's categorical ban on physician assistance to suicide -- as applied to competent, terminally ill patients who wish to avoid unendurable pain and hasten inevitable death -- substantially interferes with this protected liberty interest and cannot be sustained."