CHC solver access

[maul-esel]

This PR allows access to several CHC solvers via a unified API. Solvers can be invoked via the IChcScript interface, or by including the ChcSolver plugin in the toolchain (the plugin just calls IChcScript methods internally). We have some support for extracting models (if SAT) and derivations or unsat cores (if UNSAT).

• eldarica, accessed via the scala API. Supports models, derivations and UNSAT cores.
• golem, executed via command line. Supports models. Note: Currently, there is no Windows build of golem. I've spent some time looking into it, and it seems like it might be achievable, but no guarantees.
• Z3 / spacer, via the SMT-LIB2 interface (Script). Supports models and UNSAT cores.
• TreeAutomizer, by starting the CEGAR loop directly. Supports UNSAT cores.

TODO

• [x] reorganize the regression tests: drop tests that time out or use features unsupported by the solvers (we don't intend to test the solvers here, only their integration)

[maul-esel]

Setting unknownIsSuccess to true is indeed problematic, because we still want to test TreeAutomizer. Also, for unsupported features, exceptions are thrown -- which shouldn't count as UNKNOWN.

[maul-esel]

@danieldietsch: Sorry:

I still don't understand your issue with merge commits, though.

[danieldietsch]

@maul-esel ... we can discuss this, but if you like you can read, e.g., https://www.atlassian.com/git/tutorials/merging-vs-rebasing