Consider 'session' objects

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This could take several forms, but the goal is to allow 'sub' client/server interfaces for ongoing, scoped communication. One example would be passing some callback-like type to a method which can then be invoked several times. Another would be to allow client/server methods to return non-void interface types, to be asynchronously created on the other end of the wire, and interacted with in the same way as a client/server type.

Combining both ideas give us the ability to easily spin up matched pairs of interfaces. Using only one of the two requires that the client/server have matched pairs of methods that have to be invoked to correctly start this situation.

In any case, these session types must have some kind of close method to shut down the corresponding instance on the far side of the wire. Failure to do so would result in a memory leak (which is why Callbacks are single-use-only).