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

[Stefan Monnier]

> You're a mathematician, aren't you?  I'm not speaking of mathematical
> (i.e. absolute) necessity here, but of a practical necessity.

I agree with the practical necessity to narrow.
I strongly disagree with the necessity to make re-widening
technically impossible, I find it fundamentally incompatible with
Emacs's philosophy and can't see any practical justification here.
Just narrow and make sure jit-lock.el and font-lock.el don't
accidentally widen it.  Any other accidental widening should be
considered as a bug anyway (and we could even easily cook up some ad-hoc
advice to try and detect those cases for people like me who lke to run
their Emacs with lots of extra runtime debugging checks).


        Stefan