replace restore_size_trail by more generic restore_vector

other updates:
- change signature of advance_qhead to simplify call sites
- have model reconstruction replay work on a tail of dependent_expr state, while adding formulas to the tail.

src/ast/simplifiers/solve_eqs.cpp

@@ -237,7 +237,7 @@ namespace euf {
             save_subst(old_fmls);
         }
 
-        advance_qhead(m_fmls.size());
+        advance_qhead();
     }
 
     void solve_eqs::save_subst(vector<dependent_expr> const& old_fmls) {