jwt, paseto: migrate to new Json API (Codec. submodule, of_string, structured errors)
- Codec combinators moved to Json.Codec.* (Object, string, int, etc.).
- Json_bytesrw sublib collapsed into core: Json.of_string / Json.to_string.
- Json.Error.t returned by decode/encode; wrap with Json.Error.to_string when
feeding into library-specific string error types.
- Dune files drop the json.bytesrw dep (rolled into json).