bug#64535: 30.0.50; Spurious newlines in `prin1` output

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 08 Jul 2023 15:19:13 -0400
> From:  Stefan Monnier via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> `prin1` never inserts newlines unless they're within strings (and that
> can be controlled with `print-escape-newlines`), right?  Right?
> 
> Nope: it does insert newlines inside char-table when reaching the 3rd
> level of subtables.  This was done to work around the long lines problem
> in redisplay (bug#2866), but it's far from the only case where `prin1`
> can emit a long line, and we've significantly improved our handling of
> long lines.
> 
> Can we get rid of this quirk now, please?

I'm okay with getting rid of it, with two conditions:

  . removing newline insertion in the scenario of bug#2866 leaves
    Emacs sufficiently performant, even in the unoptimized build, and
  . we will still insert a newline if long-line-threshold is nil

IOW, I don't want us to bring back regressions as result of this
cleanup.

Thanks.