If a user has state variables called both x and x.next in their BTOR2 file, Pono currently throws an error, because TransitionSystem.make_statevar creates two solver symbols, an identically-named one for the current value of the state variable, and one with ".next" appended for the next value. This PR changes the suffix to be something that is less likely to clash with actual signals. It is still possible to cause a crash deliberately, but the only way to avoid that would be to make the variable names less readable and using something like an UUID.