Renamed flag macro names as well:
`MRB_FLAG_IS_FROZEN` -> `MRB_FL_OBJ_FROZEN`
`MRB_FLAG_IS_PREPENDED` -> `MRB_FL_CLASS_IS_PREPENDED`
`MRB_FLAG_IS_ORIGIN` -> `MRB_FL_CLASS_IS_ORIGIN`
`MRB_FLAG_IS_INHERITED` -> `MRB_FL_CLASS_IS_INHERITED`