[struct_pack][fix] set std::bitset as unportable type

[poor-circle]

Why

std::bitset is hard to cross platform between big-endian & little-endian.

What is changing

Set it as unportable type now.

User can define marco STRUCT_PACK_ENABLE_UNPORTABLE_TYPE to enable std::bitset.

Example

#define STRUCT_PACK_ENABLE_UNPORTABLE_TYPE
#include<ylt/struct_pack.hpp>
int main() {
  auto buffer = struct_pack::serialize(std::bitset<114>{});
  return 0;
}