DRAFT: Changing Names of Built-in classes

[Bobrow.pa@Xerox.COM]

The class associated with a particular symbol can be changed by using
setf with the accessor symbol-class.  It is undefined what happens if
the user attempts to change the class associated with a symbol that is
defined as a type specifier in CLtL.  
e.g.  (SETF (SYMBOL-CLASS 'CONS) ...)