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/bit_blaster.cpp

@@ -61,7 +61,7 @@ void bit_blaster::reduce() {
     }
     m_rewriter.cleanup();
     
-    advance_qhead(m_fmls.size());
+    advance_qhead();
 }