add a test: overload global operator delete

and move folder STL/memory to syntax/memory.
add a test: change access control by using expr.

用 using 改变派生类对基类成员的访问控制

基类的 protected 成员，在派生类中通过 using 可将其变为 public
成员供外界访问。
基类的 public / protected 成员通过 private 继承变为派生类的私有成员，同样可通过 using 将其变为 public 成员。
add a test: ctor of virtual base

ctor of virtual base should be called from every derived class's ctor.