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

[Gregory Heytings]

> How 'bout we do the reverse then: set the narrowing, but let-bind a 
> variable to indicate that we're inside a line-length-induced narrowing, 
> together with the previous narrowing bounds, so jit-lock or its clients 
> can undo the narrowing when needed?

That's not possible: the narrowing is (really) locked (with un uninterned 
symbol), it cannot be undone.  What would be possible would be to add an 
optional "unlock" argument to widen.  But somehow I don't think that would 
be TRT, as mode authors who now do a (widen) would simply take the habit 
to write (widen t) instead, and the same problems would surface again.

BTW, my tests show that syntax-ppss can be rather slow, when the file is 
large enough (say 1 GB).  I didn't look at what it does, but is it not 
possible to design a version of syntax-ppss that would approximate, with 
some heuristics, what syntax-ppss does, but on a smaller chunk of the 
buffer?  For example, I'd guess that '"' immediately followed by an 
alphanumeric character most likely starts a string, and '"' immediately 
preceded by an alphanumeric character most likely ends a string.