Interface for example evaluation cache utilities (#3726)

Interface for example evaluation cache utilities (#3726)

This adds interfaces in synth_conjecture for getting an ExampleEvalCache, per enumerator.
It also adds a specialization `checkRefinementEvalLemmas` of `getRefinementEvalLemmas` in the cegis module, which does evaluation on CEGIS refinement lemmas without structural generalization.  This function will be used as an alternative to `getRefinementEvalLemmas` for fast enumerators.

The next PR will update all utilities to use ExampleEvalCache instead of SygusPbe for evaluating examples.