Add class `TopSet` for sets with ⊤ element

This closes issue #49 (closed). Some remarks:

• It's called TopSet, which corresponds with SemiSet.
• We now have a generic SetUnfoldElemOf for any TopSet.
• The Hint Resolve coPset_top_subseteq : core no longer works because the lemma is generic in a type class. AFAIK it is only used for solve_ndisjoint, so I added a specialized version to the ndisj hint database. We already have specialized versions of other set lemmas there.