structure_to_tuple accepts scalars

I don't know if this is intended behavior, since I couldn't find anything in the docs. I was expecting that this:

```
auto t = pfr::structure_to_tuple(42);
```

Produced a compile-time error. As long as the argument is a scalar, it's accepted, producing a `std::tuple`, in the above case. From the `static_assert`s in the code, this may be intended, but is pretty surprising behavior.