Add support for a compare-fn in add-to-list

[Stephen J. Turnbull]

Brian Palmer writes:

 > -  (if (member element (symbol-value list-var))
 > +  (if (if (not compare-fn)
 > +         (member element (symbol-value list-var))
 > +       (member* element (symbol-value list-var) :test compare-fn))

How about

    (if (member* element (symbol-value list-var)
                 :test (or compare-fn #'equal))

instead?  Or for efficiency (does that ever matter here?)

    ;; member and memq are implemented in C
    (if (cond ((eq compare-fn #'eq) (memq element (symbol-value list-var)))
              ((null compare-fn) (member element (symbol-value list-var)))
              ((member* element (symbol-value list-var) :test compare-fn)))

Do we have any use cases for :test functions other than member?