Implement `Zeroize` for `NonIdentity<AffinePoint>`

[daxpedda]

The way we implement Zeroize for NonIdentity is to set it to the generator point. To that end we rely on the Group trait to get us that generator point. Unfortunately this means Zeroize isn't implemented for NonIdentity<AffinePoint> because they don't implement Group.

This could be solved with specialization on the trait PrimeCurveAffine, but unfortunately Rust isn't there yet.

I propose the following solutions:

1. We split NonIdentity into two types (see Introduce NonIdentityPoint #1170).
2. Introduce a new trait to get the generator point (similar to Shared identity point trait zkcrypto/group#41).

(feel free to close the issue if its not considered worth the effort)