Integrate atomic type constraints with unification

[michaelballantyne]

It should help us avoid some occurs checks. Consider:

(define-relation (foo x y)
  (symbolo x)
  (== x y))

In the case where y is a very large list, we'll do the unification and the expensive occurs check before rechecking the symbolo constraint and discovering it is not satisfied. If we integrate atomic type constraints with unification and the substitution, the failure can be discovered immediately during unification.