change default value for #ptr{} field types to a proper null value.

fix regression in ecapnp_compiler after the change to
how union values of type void are returned.
Major re-design of RPC system.

Now that I got it working a first time, I finally have some idea
of what I'm trying to do..
fix promises.

the sample calculator application works except when
using the C++ client to talk to the Erlang based server.