Make serde an optional dependency

[uint]

For a user using Sim as a Rust library, serialization might be unnecessary and pulling in serde, serde_json and serde_yaml might not make sense. It would probably make sense to put all serialization/deserialization stuff behind a serde feature.

[ndebuhr]

Agreed. I'll look into this once the Model strategy has settled (e.g., #2) .