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

[Stephen J. Turnbull]

Brian Palmer writes:

 >    (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?

Hm, that's a good point.  On the other hand, add-to-list is a pretty
userland kind of function.  Maybe it should be moved later.  Also,
member* is defined as a compiler macro; it would be nice if that could
be used.

 > > 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.

A "use case" is real code, not an accident of API.  I'll take that as
a "no". ;-)