Some renaming

This is a pure refactor, tidying up some inconsistent naming:

   isEqPred          -->  isEqClassPred
   isEqPrimPred      -->  isEqPred
   isReprEqPrimPred  -->  isReprEqPred
   mkPrimEqPred      -->  mkNomEqPred
   mkReprPrimEqPred  -->  mkReprEqPred
   mkPrimEqPredRold  -->  mkEqPredRole

Plus I moved mkNomEqPred, mkReprEqPred, mkEqPredRolek
  from GHC.Core.Coercion to GHC.Core.Predicate
where they belong.  That means that Coercion imports Predicate
rather than vice versa -- better.