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/muz/spacer/spacer_util.cpp

@@ -891,7 +891,7 @@ namespace {
             for_each_expr(cs, fml);
             return false;
         }
-        catch(found) {
+        catch(const found &) {
             return true;
         }
     }