PPX in iocaml

[hcarty]

Extension points and attributes do not seem to work properly from a non-JS iocaml environment. For example:

#use "topfind"

#require "ppx_deriving_yojson"

type t = {
  x : int;
  y : int;
} [@@deriving yojson]

Those cells don't end up defining the expected to_yojson/of_yojson functions.

This is with the latest from opam as of April 14, 2016.

[andrewray]

The kernel needs to run Pparse.apply_rewriters. I've tried to add it, but it causes strange crashes elsewhere after a ppx is enabled. Will need to have a closer look into why this is happening.