Experiment with state merging

[gustavo-grieco]

Manticore used to do state merging when performing symbolic execution. While this was never implemented for EVM, it could be a good idea to consider it for hevm.

[msooseth]

While on paper this may be interesting, I think this is currently not the most important thing to work on. I'd like to make sure we can deal with loops, recursion, and unknown code (i.e. CALL to transfer funds) at least to some extent before we start making solving faster. I find that solving faster is not the most important issue right now. Most of the time, we can just throw more CPU at the problem. And if the problem is unsolvable, because of complicated maths (mod, mul, etc over 256b integers) then state merging won't help anyway.