[Development] QHash for Qt 6

[Ulf Hermann]

> And my point was that Qt shouldn't _really_ bother reinventing something 
> already easily available elsewhere. Usage of a "fast hash" internally to 
> Qt doesn't mean we provide that API to our user (hello, QFlatMap; or 
> std::vector usages all over Qt internals.).

The fun starts when the user works with Nice3rdPartyHash in their 
application, then transforms it into QHash to use our public API, which 
we then transform into QReallyGreatHash for internal usage.

2c