CLOS, MOP and functions: Add example on how to make a class funcallable

[Ambrevar]

Making a class funcallable can be very useful, for instance when implementing a handler type, or a command type.

You can find an example in the CLISP manual: https://www.gnu.org/software/clisp/impnotes/mop-slot-access.html#mop-sa-funcallable

The portable version uses closer-mop.