[struct_pack] refact marco

[poor-circle]

Why

What is changing

1. Instead struct_pack macro regist code by ylt::refl.
2. Add document

Example

breakchange:

struct_pack_friend_decl is removed. now we can regist macro instead of struct:

classs Hi {
   int a;
   std::string b;
   YLT_REFL(Hi,a,b)
};

[github-actions]

for detail, goto summary download Artifacts base-ylt-cov-report(base commit coverage report) and ylt-cov-report(current pull request coverage report)

[github-actions]

for detail, goto summary download Artifacts base-ylt-cov-report(base commit coverage report) and ylt-cov-report(current pull request coverage report)