[struct_json, struct_yaml][feat]support escape during serialization

[bbbgan]

Why

#577

support escape during serialization

What is changing

Example

#include <cassert>
#include <iostream>
#include "ylt/struct_json/json_reader.h"
#include "ylt/struct_json/json_writer.h"

struct person {
  std::string name;
  int age;
};
REFLECTION(person, name, age);
void test_escape_serialize() {
  person p{"小\t强", 20};
  std::string ss;
  struct_json::to_json(p, ss);
  std::cout << ss << std::endl;   \\ output:  {"name":"\u5C0F\t\u5F3A","age":20}
  person p1;
  struct_json::from_json(p1, ss);
  assert(p1.name == p.name);
}

[poor-circle]

TODO: struct_xml