Bring back the Isabelle integratation for the backend verification (maybe?)

[dzufferey]

At some point, there was some effort to add Isabelle as a possible backend "solver" for the verification. Unfortunately, that was never completed and I never managed to port it to more recent versions of Isabelle. I started with sample code for Isabelle 2016 and never managed to move to newer version.

For the moment, I removed that part.

The last commit with the Isabelle code is 430d2b7.