QuickFix: remove occurences of _ in set theory definitions causing identifier errors

Changed names in definitions of standard set theoretic functions and predicates, e.g. power_set to powerSet to fix an error caused by Identifier checking for occurrences of _ as a separator. Relevant references (though in some currently commented proofs) were also changed.

Side benefit: the string versions now match the Scala versions of the definitions :partying_face: .