Forward slashes don't need to be escaped in JSON.

This isn't related to JSON-RPC but is a fix needed for the Ekam language server.

This implements JSON-RPC 2.0: https://www.jsonrpc.org/specification

In particular, it should not be necessary to specify a void union member's value explicitly. The discriminator should be sufficient. It should also be permitted to omit default values as long as the descriminator specifies the variant.

(Also make sure that external struct-unions are tested.)

This is particularly needed since JsonValueHandler is re-registered every time a JsonValue field is seen.

The code was init()ing the union field when the discriminant was seen, but this only works if the field type is a struct or group. Instead we store the field schema and initialize it when we see the value later.

This avoids the need for a weird struct-as-namespace pattern inside json.capnp.

Features:
- Rename any field or enum value for JSON purposes.
- Flatten structs/groups into their parent object, possibly with a prefix.
- Assign a special discriminant field for unions, so that union members can be flattened without ambiguity.