bug#56682: Fix the long lines font locking related slowdowns

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> That's not possible: the narrowing is (really) locked (with un uninterned
> symbol), it cannot be undone.  What would be possible would be to add an
> optional "unlock" argument to widen.  But somehow I don't think that would
> be TRT, as mode authors who now do a (widen) would simply take the habit to
> write (widen t) instead, and the same problems would surface again.

Emacs is not in the business of preventing people from shooting
themselves in the foot.  If we need this narrowing to be enforced
because Emacs would otherwise crash, then it's OK, but if not, then we
*should* provide a way to undo it.

> BTW, my tests show that syntax-ppss can be rather slow, when the file is
>  large enough (say 1 GB).

No doubt.  But it's no slower with long lines than with short lines.
[ Note that it calls `syntax-propertize` internally, which would need
  to be considered separately since `syntax-propertize` does work
  line-by-line, and should hence obey the narrowing.  ]


        Stefan