structure_to_tuple accepts scalars

[anarthal]

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<int>, in the above case. From the static_asserts in the code, this may be intended, but is pretty surprising behavior.