Langchain

Work in progress interface for building langchain framework tools. Will attempt 1-1 correspondence with the python langchain framework (including serialisation)

Objectives:

• [ ] 1-1 correspondence with python langchain framwork interface
• [ ] Serialisation / Deserialisation of langchain objects

Non-objectives:

• [ ] Reference / concrete implementations of various modules. Those are delegated to other crates
• [ ] Visualisation, UI tools