bug#27584: 26.0.50; alist-get: Add optional arg TESTFN

[Tino Calancha]

On Thu, 6 Jul 2017, Stefan Monnier wrote:

> > 1. In my patch `assoc-predicate' is a defsubst.
> >    Should does exit at all?
> >    If yes:
> >       *) should be a defun instead?
> >       **) should be named `assoc-predicate' or differently?
>
> It's been called cl-assoc so far ;-)
Some day your dream will be fulfilled, and `cl-lib' will be preloaded at
startup.  Then, we will not need things like `assoc-predicate'.