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/tab/tab_context.cpp

@@ -415,7 +415,7 @@ namespace tb {
             try {
                 quick_for_each_expr(p, t);
             }
-            catch (non_constructor) {
+            catch (const non_constructor &) {
                 return false;
             }
             return true;