Fix GH-15968: Avoid converting objects to strings in operator calculations.

[SakiTakamachi]

fixes #15968

As Gina commented, there was a problem with the function I was using.
#15968 (comment)

There are almost no cases where want to treat a stringable class other than the Number class as a string in operator calculations, so modify it so that the object is not converted to a string.

[Girgias]

This makes sense to me but lets see if @cmb69 has any opinions about this.

[SakiTakamachi]

Thanks!

I plan to include this in RC1. At least it's better than leaving it as it is now.

[cmb69]

Well, in coercive typing mode I would expect a class implementing __toString() to be silently converted to string, if a function is declared to accept a string. I'm aware that GMP doesn't behave that way, what I consider a bug, but since it's works like this for a long time, I won't fight it.

[SakiTakamachi]

hmm...

For now, I will adopt this PR, but it might be worth considering making a "bug fix" in the next version or later, including the GMP issue.

edit:
The basic behavior of the operator doesn't seem to take __toString() into account.
https://3v4l.org/FkZLC