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

[Gregory Heytings]

> > And syntax-ppss's speed depends on the rules applied by the particular 
> > major mode. Those could be sped up. Some optimization of this 
> > function's speed is not out of the question either.
>
> They would be more than welcome.  In fact, without such optimizations, 
> it would be unreasonable to do what the patch does.

To clarify: it should become at least six times faster.  That would mean 
that opening a large file would take 3N seconds instead of N seconds, 
which would be okay.