Avoid duplicate lemmas in datatypes (#3310)

Avoid duplicate lemmas in datatypes (#3310)

We previously were sending e.g. dt.size >= 0 lemmas when size terms are pre-registered, which can happen multiple times in a user context. This ensures we cache whether a lemma is sent in a user-context dependent way in the datatypes solver. This ensures we don't send the same lemma twice for dt.size >= 0 lemmas.