Binary operators don't work for __r*__ methods

[g-bauer]

The implementation as presented here:

#[pyclass]
struct RhsArithmetic {}

#[pyproto]
impl PyNumberProtocol for RhsArithmetic {
    fn __radd__(&self, other: &PyAny) -> PyResult<String> {
        Ok(format!("{:?} + RA", other))
    }
}

leads to the following behaviour when called from python:

# rhs is an instance of RhsArithmetic 
1.0 + rhs         # error:  unsupported operand type(s) for +: 'float' and 'RhsArithmetic'
1.0.__add__(rhs)  # error: NotImplemented
rhs.__radd__(1.0) # works as intended

while we'd expect that 1.0 + rhs should fall back to rhs.__radd__(1.0) which is not working.

From the disussion in gitter with @kngwyu :

We have to fill nb_add field in PyNumberMethods if the object has radd.