of fiber body receive first yield values.
e.g.: Fiber.new{|x| p x}.yield(:ok) #=> :ok
* cont.c: rename rb_context_t#retval to rb_context_t#value.
* test/ruby/test_fiber.rb: ditto.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@12425 b2dd03c8-39d4-4d8f-98ff-823fe69b080e