[struct_pack] support non-aggregate type

[poor-circle]

Why

struct_pack reflection system only support aggregate type now. We need support non-aggregate type, and allow user defined what field they want to serialize.

This can fix those issues:
#84 #304 #346

What is changing

add macro STRUCT_PACK_REFL(Typename,fieldName1,fieldName2...) and STRUCT_PACK_FRIEND_DECL(Typename)

Example

namespace test {
class person : std::vector<int> {
 private:
  std::string mess;

 public:
  int age;
  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, age, name);
}

namespace example2 {
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) {}
  STRUCT_PACK_FRIEND_DECL(person);
};
STRUCT_PACK_REFL(person, age, name);
}  // namespace example2

namespace example3 {
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());
}  // namespace example3

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