Consider using Rkyv for serialization

[Wiezzel]

We need to hack protobuf to achieve deterministic serialization, e.g. here.

Rkyv is a zero-copy serialization library for Rust. It could be a viable alternative to protobuf.

• Has deterministic serialization,
• Is significantly faster than prost (benchmarks),
• Has lower memory footprint, because it supports string re-use (shared pointers).

[Wiezzel]

It's rust-only and is not fully specified. Therefore, rewards bot (written in JS) would have trouble validating query log signatures. Borsh seems to be a better candidate.