consolidate freeze functionality into dependent_expr_state

rename size() to qtail() and introduce shortcuts
ensure tactic goals are not updated if they are in inconsistent state (because indices could be invalidated)

src/ast/simplifiers/seq_simplifier.h

@@ -70,6 +70,7 @@ public:
                 break;
             collect_stats _cs(*s);
             s->reduce();
+            m_fmls.flatten_suffix();
         }      
     }