Cleanup ppx API.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14180 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Adapt js example.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14181 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02