Unify SAT solver results on timeout

[iguerNL]

On timeout, SAT solvers (probably) return Unknown {env; timeout_reason}, where timeout_reason is one of {Assume, ProofSearch, ModelGen}

type timeout_reason =
    | NoTimeout
    | Assume
    | ProofSearch
    | ModelGen

• Maybe still catch Util.Timeout in frontend.ml, but add some documentation on "in which cases it may happen or not"
• For All-models and interpretation options, do something different if timeout_reason=Assume (in which case, the solver was not able to assert all the context before trying to solve the problem)