QHash heterogeneous lookup depends on C++20 concepts

The heterogeneous lookup feature is conditional on __cpp_concepts, "author's privilege" and "C++17 costs extra". But that precludes ourselves from using the feature, and it's one that's been high on the list of wishes for QHash.

So the feature should be rewritten in C++17 (enable_if). Minor differences to the semantics of the concepts used before are ok, provided they don't stand in the way of porting to C++20 concepts eventually.

Acceptance criteria:

• QHash heterogeneous lookup feature is usable from C++17 builds
• docs and tests updated