P3948R1: `constant_wrapper` is the only tool needed for passing constant expressions via function arguments

**Link:** https://wg21.link/P3948R1

Feature-test macro (updated):
```c++
#define __cpp_lib_function_ref                      202603L // freestanding, also in <functional>
```

Note that [P3961R1](https://wg21.link/P3961R1) also bumps this feature-test macro.