My bad.  The document is clearly broken.  Maybe I pressed my delete key
too much. [ci skip]

Must not be a bad idea to improve documents. [ci skip]

expose some C-APIs to try to make ractor utilities on external gems.

* add
  * rb_ractor_local_storage_value_lookup() to check availability
* expose
  * rb_ractor_make_shareable()
  * rb_ractor_make_shareable_copy()
  * rb_proc_isolate() (not public)
  * rb_proc_isolate_bang() (not public)
  * rb_proc_ractor_make_shareable() (not public)

"experimental_everything" makes the assigned value, it means
the assignment change the state of assigned value.
"experimental_copy" tries to make a deep copy and make copyied object
sharable.

To manage ractor-local data for C extension, the following APIs
are defined.

* rb_ractor_local_storage_value_newkey
* rb_ractor_local_storage_value
* rb_ractor_local_storage_value_set
* rb_ractor_local_storage_ptr_newkey
* rb_ractor_local_storage_ptr
* rb_ractor_local_storage_ptr_set

At first, you need to create a key of storage by
rb_ractor_local_(value|ptr)_newkey().
For ptr storage, it accepts the type of storage,
how to mark and how to free with ractor's lifetime.

rb_ractor_local_storage_value/set are used to access a VALUE
and rb_ractor_local_storage_ptr/set are used to access a pointer.

random.c uses this API.

instead of 'do not call it directly.' comment.

To make some kind of Ractor related extensions, some functions
should be exposed.

* include/ruby/thread_native.h
  * rb_native_mutex_*
  * rb_native_cond_*
* include/ruby/ractor.h
  * RB_OBJ_SHAREABLE_P(obj)
  * rb_ractor_shareable_p(obj)
  * rb_ractor_std*()
  * rb_cRactor

and rm ractor_pub.h
and rename srcdir/ractor.h to srcdir/ractor_core.h
    (to avoid conflict with include/ruby/ractor.h)