Use fast enumeration by default for Boolean predicate synthesis (#6440)

Use fast enumeration by default for Boolean predicate synthesis (#6440)

This updates the policy of when to apply smart enumeration: we do so if the grammar has ITE or admits Boolean connective terms. Previously, we applied smart enumeration for ITE and all Boolean grammars. However, this is misguided since e.g. partial evaluation unfolding has no opportunity to be effective if the enumerated terms are only Boolean literals.

This significantly improves run time on a challenge benchmark from @makaimann.