This commit adds support for embedded strings with variable capacity and
uses Variable Width Allocation to allocate strings.

[Feature #18051]

19cc24b34b0490b7c2779eec521fe0089e05f183 and fixups.

As ruby_set_debug_option() is called before ruby_sysinit(),
CRITICAL_SECTIONs are not initialized yet.

Without this patch, Ruby doesn't show ractor's information when
there is only 1 ractor. However it is hard to read the log when
some ractors are created and terminated. This patch makes to keep
showing ractor's information on multi-ractor mode.

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.

Old gcc (< 5 maybe?) warns that this variable is not initialized:

    debug.c: In function 'ruby_debug_log':
    debug.c:441:13: warning: 'r' may be used uninitialized in this function [-Wmaybe-uninitialized]
             if (r && len < MAX_DEBUG_LOG_MESSAGE_LEN) {
                 ^

I don't know if that is true, but adding "= 0" here must harm no one.