This commit adds a `capacity` field to shapes, and adds shape
transitions whenever an object's capacity changes. Objects which are
allocated out of a bigger size pool will also make a transition from the
root shape to the shape with the correct capacity for their size pool
when they are allocated.

This commit will allow us to remove numiv from objects completely, and
will also mean we can guarantee that if two objects share shapes, their
IVs are in the same positions (an embedded and extended object cannot
share shapes). This will enable us to implement ivar sets in YJIT using
object shapes.

Co-Authored-By: Aaron Patterson <tenderlove@ruby-lang.org>

Fixes id.h error during updating ripper.c by `make after-update`.

While it used to update id.h in the build directory, but was trying to
update ripper.c in the source directory.  In principle, files in the
source directory can or should not depend on files in the build
directory.

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)

This commit introduces Ractor mechanism to run Ruby program in
parallel. See doc/ractor.md for more details about Ractor.
See ticket [Feature #17100] to see the implementation details
and discussions.

[Feature #17100]

This commit does not complete the implementation. You can find
many bugs on using Ractor. Also the specification will be changed
so that this feature is experimental. You will see a warning when
you make the first Ractor with `Ractor.new`.

I hope this feature can help programmers from thread-safety issues.