Implement the occurs check

Implement the occurs check

In the writeback phase, the typechecker now checks that it isn't
replacing a type variable T with a type that contains T. It
also does an occurs check in do_autoderef in order to avoid
getting into an infinite chain of derefs.

I'm a bit worried that there are more places where the occurs
check needs to happen where I'm not doing it now, though.

Closes #768