- add struct class::class_attrs, which is designed to point to an 
  array of class_attributes that are added when the class is registered
  and removed when the class is unregistered. 
  This allows for more consolidated and cleaner definition of and
  management of attributes.

- Add struct class::class_dev_attrs to do something similarly for 
  class devices. Each class device that is registered with the class
  gets that set of attributes added for them, and subsequently removed
  when the device is unregistered.

Each array depends on a terminating attribute with a NULL name. Hint:
use the new __ATTR_NULL macro to terminate it.