RPC: Deferred improvements to internals of object system wrt delegation/inheritance

Here are some things we discussed adding to the RPC system, but decided to defer until things are more settled and we have more time:

Improved support for method delegation. Right now, it's not so practical, since the only entry point into the method-dispatch system gives you type-erased return values.
Ability to provide "inheritance" for objects. (Like method delegtion, but "if method is not found on object X, try object Y instead.")

And one more to kick around:

Support for using the RPC method-call system to call Rust methods that are not exposed via RPC. (For example, they might return Object rather than ObjectId.)

None of this is for now; none of this is an RPC blocker.