Implement generic serialization/deserialization

[ed255]

• Move all backend-specific serialization code to the backend.
• Extend the Pod trait to allow full serialization of any pod as a trait object so that serialization is fully generic.
• Implement deserialization for each pod in a generic way (following the same interface) and add a registry in the middleware to handle deserialization of any registered pod
  • Libraries that use pod2 can register new pod types to be deserialized. They can do that by calling the new middleware function register_pod_deserializer
• The frontend no longer imports the backend for deserialization and instead uses the middleware functionality for generic deserialization.
• Change the implementation of MainPod and MockMainPod pub_self_statements method to return the non-padded list of statements so that they match with the data that the PodBuilder generates. This way we can skip some data processing in serialization/deserialization.

Resolve #258

[robknight]

lgtm! I think it would be worth aligning the name of kvs and dict but otherwise all looks good.