Support conversion between struct_pack and json without field names

[romanholidaypancakes]

What happened + What you expected to happen

There are the following structures:

struct person {
  int64_t id;
  std::string name;
  int age;
  double salary;
};

Can be converted to:

[1, "name", 1, 1.333333]

Reproduction way

n/a

Anything else

n/a

Are you willing to submit a PR?

n/a

[qicosmos]

yes, you can do that:

  person p{32, "tom"};
  auto tp = struct_pack::detail::visit_members(p, [](auto&... args){
    return std::make_tuple(args...);
  });

  std::string s;
  struct_json::to_json(tp, s);

but struct_pack can reflect member names in c++20 after refactor, and the code will be more simpler, you can serialize a struct pack struct to json directly, no need transfer it to a tuple any more.