Add FastSetUnification

[magnus-madsen]

• Use FastBoolUnification as baseline.
• Copy TestFastBoolUnification to TestFastSetUnification
• Replace Term.True and Term.False by a new Term.Val(s: Set[Int]) -- this represents a concrete set.
• Add an alias Term.Bot which is just Term.Val(empty).
• Add a new term Term.Top -- this symbolically represents the universe.
• Ensure that propagateAnd is only used if a term does not contain any concrete sets (e.g. Term.Val). Otherwise it is incorrect.