Full ISA non-interference stepper #22

We can symbolically evaluate the ISA through the Stepper module in the attached branch. There are two main things to do though:

1. Attach it to an actual program.
   • This entails marking what is secret and public and doing a non-interference check.
   • We can start with a very toy program though!
   • Like binsec, we probably also need to check at every branch individually (as not to blow up the search space)?
   • Once a toy program works, we can think about loading from a bigger program.
2. It's probably going to be slow, we might need some array folding at some point. Solving this in Grisette is hard however: we need a path condition in order to do fast resolution but the solver doesn't permit this. We can think about how to solve this if step 1 is working!