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

[Gregory Heytings]

> > I have seen no effort whatsoever to make syntax-ppss significantly 
> > faster in one way or another.
>
> syntax-ppss is limited in its performance by parse-partial-sexp.
>
> Could you take a look at it? Seems like more of your area of expertise.

I already said (at least twice) that I would do that.