New port: devel/capnproto

Cap'n Proto is an insanely fast data interchange format and capability-based
RPC system. Think JSON, except binary. Or think Protocol Buffers, except
faster.

WWW: https://capnproto.org/

PR: 231500
Submitted by: Felix Hanley <felix@userspace.com.au>
Reviewed by: tobik, w.schwarzenfeld@utanet.at, ndowens@yahoo.com