atdgen: support for wrapped keys for object repr in melange

[danielmercier]

This PR implements support for wrapped keys when using <json repr="object">:

As a simple example:

type t =
   ( string wrap <ocaml t="int" wrap="int_of_string" unwrap="string_of_int">
   * int) list <json repr="object">

Really unsure if this is the correct approach as re-using the encoder/decoder for the name is the simplest solution I found. This emulates what is done in oj_emit.ml. But this changes the Encode/Decode APIs

[mjambon]

This seems like an unusual need. Wouldn't it be best served by using an adapter that rewrites the JSON object into an array of pairs? I'm worried about the maintenance burden, atdgen being already very complicated.