[spike] C++20 types in the ABI

C++20 brings several new library features that would be great to use in the Qt API, foremost among them are coroutines and std::span. Yet, both of these are in a sense type-erasure techniques, and therefore most useful across ABI boundaries.

Traditionally, Qt has defined a set of minimum supported C++ compilers and whatever their lowest common denominator was, that's what could be used in the Qt ABI, everything else, let's call is conditionally-supported, could only be used in inline API.

It is worth noting that on MSVC, inline API in exported classes already forms part of the ABI, and no-one seems to have cared.

We should, therefore, discuss whether we soften our ABI guidelines such that we can use said conditionally-supported types also in the ABI. This would allow to create non-inline APIs that use std::span and coroutines without having to wait for all compilers to support them.

This is not such a big step: We already have that, de facto, in MSVC. The only difference would be that a C++20 project would require a C++20(or later)-build of Qt. The error upon failure to comply would be unspectacular: linker not finding a symbol in the Qt library.

So, seeing as we have allowed C++latest types in our ABI de-facto in the past, I think it's time to allow it de-jure, too.