work on proof checking

- add outline of trim routine
- streamline how proof terms are checked and how residue units are extracted.

src/sat/sat_solver.h

@@ -235,6 +235,7 @@ namespace sat {
         friend class aig_finder;
         friend class lut_finder;
         friend class npn3_finder;
+        friend class proof_trim;
     public:
         solver(params_ref const & p, reslimit& l);
         ~solver() override;