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/transforms/dl_mk_elim_term_ite.cpp

@@ -57,7 +57,7 @@ namespace {
         term_ite_proc f(m);
         try {
             quick_for_each_expr(f, e);
-        } catch (term_ite_proc::found) {
+        } catch (const term_ite_proc::found &) {
             return true;
         }
         return false;