Instead of Option<&[u8]> for the various extra fields, just return a &[u8]

[raphaelcohn]

I've written lots of code to do length checks today on the various extra fields, and its unnecessarily painful. What you might want to do instead is use Option just in case libusb returns a null - it seems unlikely, as the pointer it's using is self-referential, but I wouldn't trust a third party C library to do the right thing...