Send and receive JSON.
| Automatically decodes a JSON request into a Fay record which is
| passed to
g. The handler
g should then return a Fay record (of
| a possibly separate type) which is encoded and passed back as a
| JSON response.
| If you only want to send JSON and handle input manually, use toFayax.
| If you want to receive JSON and handle the response manually, use fromFayax