[struct_xml] struct_xml::to_xml一个包含std::map<int, int>的对象，执行struct_xml::to_xml时崩溃。且不能支持struct_pack::compatible做成员变量

代码如下
```
#include
#include

#include "ylt/struct_xml/xml_reader.h"
#include "ylt/struct_xml/xml_writer.h"
#include

struct person {
std::string name;
int age;
std::list list;
std::vector vec;
std::map idx2Int;
//struct_pack::compatible c1;
};

bool operator==(const person& a, const person& b) {
return a.name == b.name && a.age == b.age;
}

YLT_REFL(person, name, list, age, vec, idx2Int/*, c1*/);

int main() {
person p{ "tom", 20 , {1,2,3}, {1,2,3}, { {1, 11}, {2, 21}} /*, 11*/ };
std::string str;

// struct to xml
struct_xml::to_xml(p, str);
std::cout << str << "\n";

// struct from xml
person p1;
struct_xml::from_xml(p1, str);
assert(p1.name == p.name);
return 0;
}
```
问题1，因为person里有成员变量std::map idx2Int，所以执行到struct_xml::to_xml(p, str);时崩溃。崩溃如下图
问题2，如果打开struct_pack::compatible c1;相关的注释，编译不通过，编译报错如下图

因为我想让结构体person同时使用struct_pack、struct_xml、struct_json，且想保证不同版本的前后兼容性。且能支持std常用的容器，如std::map。谢谢！

Start by compiling and running the supplied reproduction with ylt/struct_xml/xml_reader.h, ylt/struct_xml/xml_writer.h, and ylt/struct_pack.hpp. Trace struct_xml::to_xml(p, str) with std::map, then retry with struct_pack::compatible enabled and inspect the compiler diagnostics. Done means the reported map crash and compatible-member compilation failure are resolved without breaking the shown XML round trip.

Written by the indexing model from the issue text.