Add NonceGenerator trait and implement RollingNonce

[AndrewWestberg]

The rolling nonce on cardano is calculated for each block. It starts at the shelley genesis hash and applies each block's eta_vrf_0 value to the previous nonce and re-hashing.

Eventually, some of these rolling nonce values are used in the calculation of the epoch nonce which is required for consensus leadership checks.

[NicolasDP]

Just last two comments (that should be able to be added via the GitHub API if you wish to add them in). Otherwise I give my stamp of approval 🥇 .

Thanks so much for this great contribution to the pallas -- and to the Cardano Rust -- ecosystem.