Fix and reenable fact vs lemma optimization in datatypes (#5614)

Fix and reenable fact vs lemma optimization in datatypes (#5614)

This corrects an issue where terms internal to datatypes were not getting properly registered e.g. as part of the indices that determine the care graph, due to a context-independent cache being used (when a SAT-context-dependent one was required).

This reenables the fact vs lemma optimization in datatypes, as it is conjectured to be correct.