Adding Extra Information to Static Expressions Export

These changes are all that were needed to support an external constraint solver. It includes:

- Class information is added to s2 constants.
- Sorts are no longer exported as just strings. Instead, we export the constructors. This was done because an external constraint solver needs the type signature of a function. If support was added for tuples, we'd need this extra info as well. 
- Unification variables have a szexp associated with them. We export that information because sometimes it can be useful for the solver.
- S2Etop constructor is supported. We encounter this constructor a lot when parsing static calls to sizeof.