Draft: WIP: Lower and middle levels of Arti rpc core.
This replaces !2222 (closed), and incorporates the comments there or turns them into TODOs.
I'm going to move ahead with this version, but any comments on futher improvements to this layer would be neat.
Next steps I'll do in parallel:
- Getting a "make this connection" implementation together.
- More testing
- Sketching the FFI.