Better solver interface

src/main/scala/inox/evaluators/SolvingEvaluator.scala

src/main/scala/inox/solvers/Solver.scala

src/main/scala/inox/solvers/SolverResponses.scala

src/main/scala/inox/solvers/TimeoutSolver.scala

src/main/scala/inox/solvers/unrolling/UnrollingSolver.scala