Draft: Demonstrate that we can't populate an rpc inventory table for TorClient<R>

(This is not actually a proposal to merging anything into Arti. It's just a convenient way to communicate with @nickm.)

This branch is an attempt at "One entry per concrete type" from #837 which I think demonstrates that this isn't going to work. The problem is that inventory::submit! needs a const expression, but constants aren't allowed to inherit generics.

@nickm, LMK if you agree with my conclusion. Or, obviously, feel free to use this as a basis for a further experiments. Anyway, I will probably have a go at my alternative proposal from #837 (comment 2898755) instead.