(proof-new) Proof rule checkers run on skolem forms (#4660)

(proof-new) Proof rule checkers run on skolem forms (#4660)

Previously, the proof rule checkers were run on witness form for convenience. However, it is more flexible for the sake of the internal calculus to run on Skolem forms. The main reason is that the conversion to/from witness form does not preserve terms that contain witness. Our preprocessing steps rely on using witness terms. This design change additionally simplifies a lot of the code of the builtin proof rule checker.

Instead, witness forms can be queried on an as-needed basis, e.g. in MACRO_SR_PRED_TRANSFORM.