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

[Gregory Heytings]

> I don't think we need such a paternalistic view of ELisp programmers. 
> ELisp programmers aren't out there looking for ways to mess things up. 
> If we give them good tools that make it easy to solve the usual problems 
> with needing `widen-unlock`, they won't start using it recklessly 
> everywhere.

I'd say it is realistic, based on the observation that half of the 
programming language modes in core do use widen, which they weren't 
supposed to do.

But this discussion is leading nowhere.  If you could point out to an 
actual (or even potential) problem caused by this locked narrowing, apart 
from an occasional mis-fontification, that would perhaps help it to 
advance.