and Fiber::Core class to realize Coroutine.
* include/ruby/intern.h: declare rb_fiber_yield(), rb_fiber_resume(),
* enumerator.c: use above api.
* test/ruby/test_fiber.rb: fix and add tests for above changes.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@13130 b2dd03c8-39d4-4d8f-98ff-823fe69b080e