Clause proof printing (#2779)

Clause proof printing (#2779)

* Print LFSC proofs of CNF formulas

* Unit Test for clause printing

* Added SAT input proof printing unit test

* Fixed cnf_holds reference. Proofs of CMap_holds

There were references to clauses_hold, which should have been references
to cnf_holds.

Also added a function for printing a value of type CMap_holds, and a
test for this function.