Replace mrb_intern() func with mrb_intern2() func or mrb_intern_cstr() func.
2.Replace mrb_intern() func with mrb_intern2() func or mrb_intern_cstr() func.
Merge pull request #1364 from suzukaze/refactor-in-mrb-intern

Replace mrb_intern() func with mrb_intern2() or mrb_intern_cstr() func.