[struct_pack] STRUCT_PACK_REFL now support regist private member

[poor-circle]

Why

STRUCT_PACK_REFL can support private memeber

What is changing

1. remove macro STRUCT_PACK_FREIND_DECL, it's useless now.
2. macro STRUCT_PACK_REFL can support private memeber.
3. regist member function in STRUCT_PACK_FREIND_DECL don't need add extra ().

Example

class person : std::vector<int> {
 private:
  int age;

 public:
  std::string name;
  auto operator==(const person& rhs) const {
    return age == rhs.age && name == rhs.name;
  }
  person() = default;
  person(int age, const std::string& name) : age(age), name(name) {}
};
STRUCT_PACK_REFL(person, name, age);

class person {
 private:
  int age_;
  std::string name_;

 public:
  auto operator==(const person& rhs) const {
    return age_ == rhs.age_ && name_ == rhs.name_;
  }
  person() = default;
  person(int age, const std::string& name) : age_(age), name_(name) {}

  int& age() { return age_; };
  const int& age() const { return age_; };
  std::string& name() { return name_; };
  const std::string& name() const { return name_; };
};
STRUCT_PACK_REFL(person, age, name);

[poor-circle]

Due to some bug of clang8-11, this pr can't remove STRUCT_PACK_FRIEND_DECL, i will try to adapt two solve later.