Fixes a bug, where UnrollingSolver was interrupted but was not
stopping its internal solver, leading to a timeout not being
respected if the internal solver was stucked in a complicated
check operation. UnrollingSolver now takes a Solver with
Interruptible.

In order to do that, Leon now relies on a more recent version of
scala-smtlib, that provides a feature to kill the solver process
in a relatively clean way. Update the SMTLIBSolver code to
actually perform the kill operation on the interrupt.

- Implement smt and smt-z3/smt-cvc4 as options for --solvers

- Uses a main solver sources for verification/CLP, taking into account
  --solvers

- Implement Z3-SMT sets

- Add tests for Z3-SMT