new solver that checks intermediate counter examples

src/purescala/Definitions.scala

src/purescala/Extensions.scala

src/purescala/IterativeZ3Solver.scala

src/purescala/Trees.scala

src/purescala/Z3ModelReconstruction.scala

src/purescala/Z3Solver.scala

src/purescala/z3plugins/instantiator/Instantiator.scala