internal iteration for clause generation

[MichaelRawson]

Something a little controversial to think about.

Vampire builds massive iterator chains in order to carry out inference. However, all of them end up being consumed and passed through SaturationAlgorithm more-or-less unconditionally.

This PR adds a parallel method to e.g. GeneratingInferenceEngine that implements internal iteration rather than constructing a ClauseIterator, i.e.

  virtual ClauseIterator generateClauses(Clause* premise) = 0;
  virtual void generateClauses(Clause* premise, ClauseReceiver receive) {
    for(Clause *cl : iterTraits(generateClauses(premise)))
      receive(cl);
  }

I've re-plumbed SaturationAlgorithm to use internal iteration at clause activation.

The pay-off for all this is that writing inferences is often much more straightforward, easier to debug and possibly more performant. I've changed Factoring as a demonstration.

(also renamed SimplifyingGeneratingInference SimplifyingGeneratingInferenceEngine because it was confusing me)

This is just a proof of concept and would need more attention if we want to go ahead with it.

[MichaelRawson]

CI failure is weird, it works locally. 🤷

[MichaelRawson]

I for one greatly prefer #920. Let's keep the discussion going there.