Iterating over model values in SMTInterpol

[cheshire]

SMTInterpol's Model interface only exposes an evaluate method, and does not support iterating over model values. The current code somewhat arbitrary decides to populate the dict with the values of the boolean constraints which were pushed to the solver stack. I think we should:

• Raise the ticket with SMTInterpol to expose that interface (it's one line, the underlying implementation is a map).
• Throw an UnsupportedOperationException for now, as it is dead-easy to get the values for those boolean terms using evaluate, and such an incomplete model might be misleading (null-s should be "don't-matter's", not "everything else")

[cheshire]

I also strongly suspect that the code for function values in model was never actually run, as we don't actually use functions of boolean sort in cpachecker.

[cheshire]

Oh wait, sorry, I was wrong, the visitor extracts all variables and UFs from the asserted formulas first... OK that should be easy to implement than.

Should be also more efficient, if we do that only in the iterator, so the model creation will not have this cost.