public key: add ssz impl

[tcoratger]

Related #11

[b-wagn]

Thanks LGTM!

Only minor criticism I have is that we now use fields in the generalized XMSS file, which breaks a bit our abstractions. I wonder if it makes sense to require that encode and decode are defined for the parameter and hash type of the tweakable hash and then call those black-box here. Then move the encoding of field vectors into the tweakable hash file.

Not sure, just an idea.

[tcoratger]

Thanks LGTM!

Only minor criticism I have is that we now use fields in the generalized XMSS file, which breaks a bit our abstractions. I wonder if it makes sense to require that encode and decode are defined for the parameter and hash type of the tweakable hash and then call those black-box here. Then move the encoding of field vectors into the tweakable hash file.

Not sure, just an idea.

Okay, so I tried to fix this with the best possible abstraction using a FieldArray abstraction. We have the same one in Plonky3 here: https://github.com/Plonky3/Plonky3/blob/b1f8c020c3120d6076339aacaffc414bf23533a4/field/src/array.rs#L11 but we can't use it because, due to the orphan rule, we can't implement a trait on it outside of the Plonky3 repository. This would have forced us to create a wrapper around it, which is even worse.

With this new abstraction, it should be super easy to build other SSZ implementations in follow-up pull requests for secret key and signature objects.