“The point at which life becomes 'worthless,' and the point at which the means necessary to preserve it become 'extraordinary' or 'inappropriate,' are neither set forth in the Constitution nor known to the nine Justices of this Court any better than they are known to nine people picked at random from the Kansas City telephone directory...[therefore] even when it is demonstrated by clear and convincing evidence that a patient no longer wishes certain measures to be taken to preserve her life, it is up to the citizens of Missouri to decide, through their elected representatives, whether that wish will be honored.”