CapNProto support

[unicomp21]

Protobuf support would be nice as well, but think this will be easier and run faster.

Ideally the CapNProto schema w/ annotations can be used to codegen the await'able (fiber based) channels referenced in:
#1440

[dcodeIO]

I don't see how CapNProto, or protobuf for that matter, are going to help the compiler specifically. Once we'll need serialization, we're likely going to build an optimized domain-specific implementation, and I'd expect CapNProto or protobuf to remain third-party packages. If you are interested in building such a package independently, that'd be great of course :)