move th->fiber to ec->fiber.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59776 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
move th->fiber to ec->fiber.
rb_execution_context_t::fiber.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59776 b2dd03c8-39d4-4d8f-98ff-823fe69b080e