support calling server-side cells from client-side cells

[jaked]

right now server / client are separate worlds, and each client-side cell is its own world (i.e. it doesn't share modules with others). So it would be useful to be able to refer to a variable defined in a server-side cell from a client-side cell, and have the value of the variable fetched via RPC. Then you could e.g. fetch data server side and display it client side.

Ideally this would work with hot reloading. We could re-run dependent client cells when the server cell changes; and/or we could integrate with React etc. to hot-reload the state without re-running the whole cell. Maybe this would involve an explicit hook, e.g. useVariable(v) where v is defined server-side. At cell rewrite time we'd turn v into a reference we can use to push state changes back to the client over RPC.