Postponed jobs are registered with this API. Registered jobs
  are invoked at `ruby-running-safe-point' as soon as possible.
  This timing is completely same as finalizer timing.
  There are two APIs:
* rb_postponed_job_register(flags, func, data): register a
  postponed job with data. flags are reserved.
* rb_postponed_job_register_one(flags, func, data): same as
  `rb_postponed_job_register', but only one `func' job is
  registered (skip if `func' is already registered).
  This change is mostly written by Aman Gupta (tmm1).
  https://bugs.ruby-lang.org/issues/8107#note-15
  [Feature #8107]
* gc.c: use postponed job API for finalizer.
* common.mk: add dependency from vm_trace.c to debug.h.
* ext/-test-/postponed_job/extconf.rb, postponed_job.c,
  test/-ext-/postponed_job/test_postponed_job.rb: add a test.
* thread.c: implement postponed API.
* vm_core.h: ditto.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40940 b2dd03c8-39d4-4d8f-98ff-823fe69b080e