Add Cap'n Proto serialization format

[lthibault]

This PR adds the Cap'n Proto serialization & RPC wire format.

I move to include this in the sub-0x80 range because of the widespread popularity and usage of the format, as evidenced by:

• 7.5k stars on GitHub for the reference implementation
• Implementation of the spec in 15+ languages
• Consistent, active development of:
  • the official spec
  • the reference implementation
  • language-specific implementations

[lthibault]

In case it weighs in the decision, I am working on a Cap'nProto implementation of IPLD:
ipld/ipld#87.