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/bit_blaster.cpp

@@ -37,7 +37,7 @@ void bit_blaster::reduce() {
     expr_ref   new_curr(m);
     proof_ref  new_pr(m);
     bool change = false;
-    for (unsigned idx = m_fmls.qhead(); idx < m_fmls.size(); idx++) {
+    for (unsigned idx = qhead(); idx < qtail(); idx++) {
         if (m_fmls.inconsistent())
             break;
         auto [curr, d] = m_fmls[idx]();