hook a VM termination.  Right now, because the VM we have is
	  process global, most extensions do not deallocate resources and
	  leave them to Operating System's reaping userland processes.  But
	  in a future we plan to have multiple VMs to run simultaneously in
	  a single process (MVM project).  At that stage we can no longer
	  rely on OSes and have to manage every resources to be reclaimed
	  properly.  So it is.  For a forward-compatibility reason this API
	  is introduced now, encouraging you to be as gentle as you can for
	  your resources; that is, tidy up your room.

	* include/ruby/vm.h: ditto.

	* vm_core.h (rb_vm_struct): new field.

	* vm.c (vm_init2): initialize above new field.

	* eval.c (ruby_cleanup): trigger those hooks.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@30050 b2dd03c8-39d4-4d8f-98ff-823fe69b080e