P3774R1: Rename `std::nontype`, and make it broadly useful

**Link:** https://wg21.link/P3774R1

Feature-test macros (increased):
```c++
#define __cpp_lib_function_ref 202511L // also in
```

Start by reading the linked P3774R1 proposal to determine the requested renaming and scope. The issue only identifies the increased __cpp_lib_function_ref feature-test macro, so locate the corresponding implementation and tests after confirming the proposal's requirements. Done means the proposal is implemented consistently and the relevant feature-test coverage passes.

Written by the indexing model from the issue text.