Sketch for some options and design issues around FFI/RPC
This is just a beginning, but hopefully we can expand it with more information and options, and then use it to get everybody in our nascent FFI/RPC working group on the same page.
@eta, @Diziet, @ahf : How can I improve this? What am I forgetting?
(Let's work out the early issues on our own before we encourage more people to review this.)