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/qe/qe_term_graph.cpp

@@ -62,7 +62,7 @@ namespace qe {
             is_pure_ns::proc v(is_var);
             quick_for_each_expr(v, e);
         }
-        catch (is_pure_ns::found) {
+        catch (const is_pure_ns::found &) {
             return false;
         }
         return true;