* ext/objspace/gc_hook.c, ext/objspace/objspace.c: add new methods to
  hook GC invocation.
* ObjectSpace.after_gc_start_hook=(proc)
* ObjectSpace.after_gc_end_hook=(proc)
  Note that hooks are not kicked immediately. Procs are kicked
  at postponed_job.
  This feature is a sample of new internal event and
  rb_postponed_job API.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40970 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* properties.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40971 b2dd03c8-39d4-4d8f-98ff-823fe69b080e