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

[Eli Zaretskii]

> Date: Thu, 01 Feb 2024 03:31:10 +0000
> From:  Okamsn via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> This patch adds support for using `seq-subseq` with `setf`, as in
> 
>      ;; => [0 1 2 10 11]
>      (let ((seq (vector 0 1 2 3 4)))
>        (setf (seq-subseq seq -2) (list 10 11 12 13 14))
>        seq)
> 
> The patch adds a generic version which uses the existing `setf` support 
> of `seq-elt` and a specialized version for modifying lists.  Both 
> versions use `seq-do` to map a function over the values that should 
> replace the values in the modified sequence.
> 
> To avoid modifying more values than specified, that modifying function 
> uses a `when` condition. I'm not sure of a good way to stop `seq-do` 
> early when we know that it can stop calling the modifying function. 
> Normally, I would use `cl-block` and `cl-return`. Is it OK to use those 
> features in `seq.el`? If not, is it worth adding something like a 
> `seq-map-while` or a `seq-do-while`?

Thanks.

Nicolas, Stefan: any comments?  seq.el is preloaded, so we should
consider whether this addition is important enough to have it in seq
or elsewhere, if we think it's useful.