T* and const T* hash to different values

It turns out the one-arg-to-two-arg adapter strikes again, and causes pointers-to-mutable to hash differently from pointers-to-const.

There does not appear to be anything we can do about this until Qt 7, as we'd need to either change T* or const T* hashing. Both functions being inline, that means this change would be BiC, whatever we do.

The fix is otherwise simple: remove the adapter (and move from qHash function templates to qHash class templates, like the std does with std::hash).

TODO: check whether recent changes to the adapter function template may have caused this in the first place.