bug#68863: Add support for using setf with seq-subseq

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> Some time ago we have removed lots of or even the majority of the
> so far implemented generalized variables.
>
> The main criteria were: are they useful, and is the semantics clear.  We
> should think about whether this is the case here.

Agreed.  We should not lose track of the fact that we're talking about
"generalized variables".  Treating `(seq-subset ...)` as a "generalized
variable" (aka a "place") is stretching the idea, which we can see in the
fact that there are several different alternative ways to implement the
feature with incompatible semantics.

IOW, as a general rule a function that returns a fresh new value is
often not a good candidate for a "gv-place".  That doesn't mean we can't
accept such a change (I'm pretty sure we do have and (ab)use such
gv-places), but that it has to be weighed against its practical
usefulness.

IOW, I'd like to see existing code where we could make use of it to
simplify the code.  That might also help decide decide which semantics
would be preferable.


        Stefan