This allows {schema,rpc,rpc-twoparty}.capnp to be used without modification when
relocated outside of the Cap'n Proto codebase.

Define additional type aliases AnswerId and ImportId.  In the RPC protocol, always use the type corresponding to the sender's point of view.  In the implementation, always use the type corresponding to the local point of view.

Change capability pointers to be indexes into a separate cap list so that CapDescriptors can be interpreted on receipt rather than delaying until the application actually traverses the message.  This massively simplifies a lot of things.

Tail calls half-implemented, but I think it's time to delete all the cancellation code before continuing because it will simplify things.

In RPC protocol, rename request -> params, answer -> results.  Also fix up Join stuff in rpc-twoparty.capnp, because it was sort of wrong.

Implement two-party network.  The first RPC call over a socket took place at 2013-11-08 14:46:43 -0800 and completed successfully.