Introduce separate Prop type

[byorgey]

Bool is a subtype of Prop. Things like forall, exists create a Prop.
Things like and, or can work with both. A Prop can be
evaluated into a Bool, using a built-in primitive named something like eval, or check, (this might not terminate), or tested using a primitive like test (which terminates). Maybe test returns some special algebraic type representing possible outcomes (definitely false (with counterexample?); definitely true; probably true; probably false).

[byorgey]

• Add Prop as a supertype of Bool
• Make and, or, not, implies polymorphic
• Make special assertion operators like ==!, <!, etc. which can only make a Prop and have special behavior when reporting test failures?
• Add eval prim to turn Prop into Bool
• Add forall and exists expression forms
• Add test primitive
• get rid of Property as a separate thing