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

[Gregory Heytings]

> Before using a blunt tool like the forced-narrowing now in 
> `feature/long-lines-and-font-locking`, I think we should try and figure 
> out *why* the recipe below is so slow.

It's not a blunt tool, it's an appropriate tool to help making sure that 
Emacs remains responsive when large files are visited.  Think of it as 
POSIX's ulimits.  Allowing fontification-functions to search for 
arbitrarily complex regexpes in an arbitrarily large buffer, each and 
every time they are asked to to highlight a small chunk of the said 
buffer, is a recipe for disaster.  If for some reason modes really need to 
go through to the whole buffer to decide which highlighting to use, they 
should to do so outside of fontification-functions, and ideally once, for 
example, when the file is loaded.

(Note that at the moment that tool is enabled only when files with long 
lines are visited.)