Add derive rkyv Serialize/Deserialize

[jonathanpwang]

As described in #19, currently the Serialize, Deserialize implementations for bn256::{Fr, G1Affine} are using the human-readable (aka to_repr() implementations) format. One should remember that this means you should NOT use Serialize, Deserialize implementations for anything where performance is critical (such as serializing/deserializing proving keys).

Currently we use bincode with the derived Serialize, Deserialize on Snark, but since snark proofs are pretty small it should be inconsequential.

We should add custom derive for rkyv::{Serialize,Deserialize} and possibly also bincode::{Serialize,Deserialize} so we can benchmark/compare different serde strategies for proving keys: axiom-crypto/halo2#7