Implement arithmetic operators with a macro

[Antonio95]

We realised the previous implementation of operators *, /, - for owned types (instead of references) broke some things. For instance, this line in poly-commit stopped compiling.

In the specific example given, the reason for the new failure is that, when operators were only implemented for &poly / &poly, the compiler implicitly converted the call f_poly.div(&z_poly); into (&f_poly).div(&z_poly);. But after we had added poly / poly, the compiler was not clever enough to add the reference and complained that poly / RHS should have RHS of type poly (not &poly).

Long story short, the best way to ensure the new operator definitions are not a breaking change is to implement poly / poly, poly / &poly and &poly / poly (all of them falling back to the honestly defined &poly / &poly). Since that had to be done for several operators, a macro seemed like the best approach.