How does a once-great nation survive when its leader is unhinged? That question is, um, perhaps less academic than it once was. So you might feel a few uneasy twinges of recognition as you sit on Boston Common watching the title figure of “Richard III’’ rage and lie and scheme and generally manipulate the mechanisms of governance in pursuit of power and personal enrichment Of course, Shakespeare wanted you to feel uneasy, wanted you to consider how unsettlingly simple it is for Richard to impose his will and gain the British throne by constructing a new reality — alternative facts, you might say — and persuading others to buy into that distorted vision.