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

[Brian Palmer]

   (if (member* element (symbol-value list-var)

>:test (or compare-fn #'equal))


Since subr.el is dumped before cl.el, I wanted to make sure that if anything
pre-CL did call add-to-list, it'd at least work if they avoided the :test
case. But nothing seems to, so I suppose this would be safe?


> 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?
>

Emacs also seems to offer a memql, but I'm not sure how much use it is other
than completism.

As for :test, it's also used in make-hash-table.
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