impl Op<Quantity<...>> for Quantity<..> is already complete. Add the following reference implementations:

• impl<'a, 'b> Op<&'a Quantity<...>> for &'b Quantity<..>
• impl<'a> Op<&'a Quantity<...>> for Quantity<..>
• impl<'a> Op<Quantity<...>> for &'a Quantity<..>