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

[Stefan Monnier]

> The problem is that this is, as I said, slow.  On my laptop, opening a 1 GB
> file takes about 6 seconds.  The call to syntax-ppss adds 70 seconds, so
> opening a large file becomes an order of magnitude slower (13 times slower).

It's meaningless to talk about the time taken by `syntax-ppss` without 
specifying
the major mode that was in use.
You might also want to compare to the time to run

    (parse-partial-sexp (point-min) (point-max))

which is a kind of "speed of light" for `syntax-ppss`.


        Stefan