Decouple our domain objects from `mina-rs` serialization types for mainnet

[trevorbernard]

This coupling isn't ideal if we want to support berkeley and the hard fork. I propose we create our own domain objects and add a conversion from mina-rs serialiation types to our own domain objects. This enables us to create conversions for the berkeley precomputed blocks and other sources of blocks like ExternalTransitions.

This also enables us to replace mina-rs serialization with our own hermetic serialization implementation.