Draft: WIP: Sketch out APIs for lower and middle levels of arti rpc lib.
This commit starts to sketch out some ideas for arti-rpc-client-core
as we'd discussed. It's nowhere near ready for merge; I'm just uploading it so that you can have a look at the APIs. I don't expect CI to pass.
If you have any thoughts about the issues I've marked with XXX, that would be helpful.