v0.3.0

This release adds new engine to the supporting backend: IMC, the original interpolation-based model-checking algorithm from McMillan.
It was contributed by @BritikovKI.

Additionally, with this release, the Spacer engine now also produces proofs of unsatisfiability.
The output format of the unsatisfiability proof has been changed and now follows more closely the output format as printed by Eldarica.