Audit task tacker

[drskalman]

Epic issue to track remaining tasks related to the audit.

Higher priorities

• Remove stale Serilizations comments from single.rs.
• Remove 383 comment.
• Deal with comments of Item 9 of https://github.com/paritytech/srlabs_findings/issues/677 (verifier, single, nugget)
• Define SisterGroup<E: EngineBLS> = CurveGroup + PrimeGroup<ScalarField = E::Scalar> + SerializableToBytes.

Optionals:

• Check if batching for two elements is worth it #87
• Implement Serde
• Audit findings related to experimental part (at least make parallel issue for them in this repo if not resolved.)

[mertwole]

• Define SisterGroup<E: EngineBLS> = CurveGroup + PrimeGroup<ScalarField = E::Scalar> + SerializableToBytes.

That comes out to be more verbose, so not rational