JSON encoding/decoding

[zenhack]

The reference implementation defines a JSON encoding of capnproto values; it would be nice for us to generate code to use this encoding, probably in the form of instances of the type classes from the aeson package, for the types in the high-level API output.

Note that the reference implementation ships with a json.capnp (which we include under core-schema/), which defines some annotations used to adjust the encoding. An implementation of this feature should respect those.