[Optimization?] signature key stores affine points

[mrain]

In primitives/src/signatures/bls_over_bn254.rs.
Current Eq, Ord of the signature key is making some non-trivial conversion from a projective point to its affine representation.

impl Ord for VerKey {
    fn cmp(&self, other: &Self) -> core::cmp::Ordering {
        self.0.into_affine().xy().cmp(&other.0.into_affine().xy())
    }
}

In case of many expected comparisons and equality checks, e.g. when use the VerKey to index an BTreeMap, we could optimize the performance by storing its affine repr by default.