* cont.c : Introdule ensure rollback mechanism. Please see below.

	* internal.h (ruby_register_rollback_func_for_ensure): catch up above change.
	  Add rollback mechanism API.

	* vm_core.h (typedef struct rb_vm_struct): catch up above change.
	  Introdule ensure-rollback relation table.

	* vm_core.h (typedef struct rb_thread_struct): catch up above change.
	  Introdule ensure stack.

	* eval.c (rb_ensure): catch up above change.
	  Introdule ensure stack.

	* hash.c : New function for rollback ensure, and register it to
	  ensure-rollback relation table. [ruby-dev:47803] [Bug #9105]

	Ensure Rollback Mechanism:
	A rollback's function is a function to rollback a state before ensure's
	function execution.
	When the jump of callcc is across the scope of rb_ensure,
	ensure's functions and rollback's functions are executed appropriately
	for keeping consistency.

	Current API is unstable, and only internal use.

	ruby_register_rollback_func_for_ensure(ensure_func,rollback_func)
	This API create relation ensure's function to rollback's function.
	By registered rollback's function, it is executed When jumpping into
	corresponding rb_ensure scope.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43688 b2dd03c8-39d4-4d8f-98ff-823fe69b080e