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/fix_dl_var_tactic.cpp

@@ -226,7 +226,7 @@ class fix_dl_var_tactic : public tactic {
                 }
                 return most_occs();
             }
-            catch (failed) {
+            catch (const failed &) {
                 return nullptr;
             }
         }