"Because the insanity defense is a key part of our criminal-justice system, which is founded on the belief that people normally choose whether or not to obey the law. Certain people, we have said historically, cannot make that choice, however, either because they are too young or because of severe mental retardation or mental illness. The insanity defense exists in practically every civilized country. It is not some kind of aberration, as some seem to be suggesting in the wake of the Hinckley verdict."