Support for Execution monad

Can't we massage the return type of the existing verifiable framework so that this fits into that? I'm not entirely sure why we need to duplicate this. It seems a matter of adding handling for a parametrized return type (unit for jobs and flows) and shuffling between different "either" types.

Anyway, regarding the args issue, if you think that we'll just be evaluating already defined library functions, that makes sense. I'm a little skeptical, but either way it's not a big deal.