Add Cap'n Proto deserialization

[debrouxl]

Cap'n Proto ( https://capnproto.org/ ) was created by the original author of Protocol Buffers v2 (the open-source version). The lineage is obvious, but Cap'n Proto brings a significant range of improvements, at least one of which was recently implemented in protobuf: arena allocation.
There's a Rust implementation, https://github.com/dwrensha/capnproto-rust .

[dflemstr]

This one is a bit tricky because I'd need to implement dynamic capnp parsing like I did with serde-protobuf and serde-avro. I probably can't use the existing capnp library (well, maybe the compiler part).

Would be great to get pull requests for this!