"Our decision in Roe v. Wade, ante p. 410 U. S. 113, establishes (1) that, despite her pseudonym, we may accept as true, for this case, Mary Doe's existence and her pregnant state on April 16, 1970; (2) that the constitutional issue is substantial; (3) that the interim termination of Doe's and all other Georgia pregnancies in existence in 1970 has not rendered the case moot; and (4) that Doe presents a justiciable controversy, and has standing to maintain the action."