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/bv_slice.cpp

@@ -24,7 +24,7 @@ namespace bv {
     void slice::reduce() {
         process_eqs();
         apply_subst();
-        advance_qhead(m_fmls.size());
+        advance_qhead();
     }
 
     void slice::process_eqs() {