This module allows you to declare class attributes in exactly the same way
as you declare object attributes, except using 'class_has()' instead of
'has()'. It is also possible to make these attributes immutable (and
faster) just as you can with normal Moose attributes.

You can use any feature of Moose's attribute declarations, including
overriding a parent's attributes, delegation ('handles'), and attribute
metaclasses, and it should just work.

The accessors methods for class attribute may be called on the class
directly, or on objects of that class. Passing a class attribute to the
constructor will not set it.