Add some left-out `builtin_traits` instantiations.
Add some left-out `builtin_traits` instantiations.

This would explain why a recent Visual C++ would complain at link time
that it couldn't find `string_traits<T>::from_string` where `T` is a
floating-point type: I forgot to write the explicit instantiations for
the case where `std::from_chars()` is available for floating-point
types!

Still leaves the identical errors for the non-`from_chars` case a
mystery though.

Hope this one will at least help #216.

jtv/libpqxx,jtv/libpqxx,jtv/libpqxx,jtv/libpqxx