Catch exceptions by const-reference

Exceptions caught by value incur needless cost in C++, most of them can
be caught by const-reference, especially as nearly none are actually
used. This could allow compiler generate a slightly more efficient code.

src/tactic/arith/pb2bv_tactic.cpp

@@ -1034,7 +1034,7 @@ struct is_pb_probe : public probe {
             
             return true;
         }
-        catch (pb2bv_tactic::non_pb) {
+        catch (const pb2bv_tactic::non_pb &) {
             return false;
         }
     }