replace some copies with moves

src/sat/sat_mus.cpp

@@ -208,7 +208,7 @@ namespace sat {
                 lits2.insert(*it);
             }
         }
-        lits1 = lits3;
+        lits1 = std::move(lits3);
     }
 
     literal_vector& mus::get_core() {

src/sat/sat_xor_finder.cpp

@@ -210,7 +210,7 @@ namespace sat {
                 }
                 bv.push_back(parity);
             }
-            m_parity.push_back(bv);
+            m_parity.push_back(std::move(bv));
         }
     }