bug#64536: 30.0.50; Make cl-print put buttons on ellipses

[Stefan Monnier]

>> That's also what I thought at first, but the case of `inspect.el` made
>> me realize that it's likely that just letting them use
>> `backtrace-ellipsis` button is often not the right solution anyway.
> I am pretty sure that some uses of `backtrace-ellipsis' have nothing to
> do with printing Elisp values.

Could you expand on what makes you think so?
I grepped for it in Melpa and couldn't find any use of it at all, FWIW.

> At least, there is no reason why they should be limited to.

It was rather tightly integrated with `cl-print`, tho:
the button called `backtrace-expand-ellipsis` which itself looked for
a `cl-print-ellipsis` text-property and called
`cl-print-to-string-with-limit` and `cl-print-expand-ellipsis` on it.

>>> May it be possible to create an obsolete alias for this button type?
>> And I don't think we have such aliases.
> May it be added?

Patch welcome (especially if it comes together with some kind of
obsolescence mechanism).


        Stefan