(cad solver) Add a partial check method. (#4904)

(cad solver) Add a partial check method. (#4904)

This PR extends the CAD-based solver to enable partial checks. Essentially, we only collect the first interval that is excluded for the first variable and return that one as a lemma. This does not leave a lot of choice on "how partial" the check should be, but it is fairly easy to implement and does not add additional overhead.
It also fixes some confusion in excluding_interval_to_lemma...