bug#50704: Feature request: Add ensure-list in cl-lib

[Clément Pit-Claudel]

Hi all,

There are 40+ instances of the following code pattern in Emacs' code base, and 
many more in packages:

   (if (listp x) x (list x))

It's easy to find single-line instances of it using the regexp [(]if [(]listp 
.*[(]list .

This pattern is called "ensure-list" in CL:

  (defun ensure-list (thing)
    "Returns THING as a list.

  If THING is already a list (as per listp) it is returned,
  otherwise a one element list containing THING is returned."
    (if (listp thing)
        thing
        (list thing)))

I would like to add a similar function in Emacs, since it comes up so often.  
Is that an acceptable change?  

If so:
- Which cl-*.el file should it go into, and where in that file?  
- Should it be a defun or a define-inline?  
- And once it is introduced, should the 40-some instances that could use it be 
updated, or left as-is?  
- Finally, where should it be documented?

Cheers,
Clément.