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.

- Introduce Leon strings (smt-based solvers only)
  Available in leon.lang.string._

- Print method calls properly, specialize binary ops

reactivate tests for CVC4, if available. Remove non-linear testcase that
worked only on z3 (SumAndMax)