Overlap of free variables and other types should not be empty

Problem

Intersections containing type variables and values like

(I a (Val :a))

are being simplified to

(U)

We cannot predict what a type variable overlaps with (well, technically we could use its type bounds, but let's be more conservative for now), so this intersection should be left unchanged.

Solution

Overlap is the culprit. Ensure there is always an overlap between type variables and other types.

Pull request: 82
Commit: c1cfba87