API Userdata

Userdata types may be registered with an API using the userdatas feature.

Example:

#include <primer/api/userdatas.hpp>

struct my_api : primer::api::base<my_api> {
{
  ...

  API_FEATURE(primer::api::userdatas<my_userdata1, my_userdata2, my_userdata3>, udatas_);

  ...

};

The metatables of each userdata will be created when initialize_api is called, and the permanent objects, defined by the userdata trait, if any, will be registered when we persist / unpersist.

The userdatas object itself has no data members and no nontrivial initialization.