bug#73431: Add `setf` support for `stream.el` in ELPA

[Philip Kaludercic <philipk@posteo.net>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>>> Sounds like a bug, indeed.  But I'd rather fix it by making it fail
>>> cleanly, to preserve the (current) immutability of streams (at least
>>> until we decide that there's a good reason for streams to be mutable).
>> One exception to the immutability of stream might be buffers?
>
> Sorry, I don't follow.  What do you mean by that?

Using (stream (current-buffer)) i create a stream of things in the
current buffer.  E.g. using

(seq-find
 (lambda (line)
   (and line (string-match-p "seq" line)))
 (stream (current-buffer) nil 'defun))

I can try to find the first top level definition that contains a
substring (the need to check if the value is non-nil is a bit annoying).

Being able to modify the head of a buffer-stream using setf seems like
something that could be useful, and certainly more efficient than what
many people want to do with splitting the return value of
(buffer-string).

-- 
	Philip Kaludercic on siskin