Implement Mul<&NonZeroScalar> for NonIdentity

[daxpedda]

For context: setting where bounds on traits can in some situations be "poisonous", meaning that they have to be repeated everywhere you use the trait.

E.g.:

trait Protocol {
    type NonZeroScalar;
    type NonIdentity: Mul<&Self::NonZeroScalar>;
    ...
}

As opposed to:

trait Protocol
where
    &Self::NonIdentity: Mul<&Self::NonZeroScalar>
{
    type NonZeroScalar;
    type NonIdentity;
    ...
}