P1169R4 static operator()

In addition to core language changes, the paper also requires extending deduction guides for function and packaged_task to deduce from classes with static operator().

No library feature-test macro.

The library change is meaningless and not testable as long as the core change is not implemented. Once the compilers have implemented this feature, I think we can change some CPOs, niebloids, and internally used lambdas to use static operator().