“History shows that governments sometimes seek to regulate our lives finely, acutely, thoroughly, and exhaustively. In our own time and place, criminal laws have grown so exuberantly and come to cover so much previously innocent conduct that almost anyone can be arrested for something. If the state could use these laws not for their intended purposes but to silence those who voice unpopular ideas, little would be left of our First Amendment liberties, and little would separate us from the tyrannies of the past or the malignant fiefdoms of our own age. The freedom to speak without risking arrest is ...