Ppx syntax extension

So, I think it's ready now. There is still a small opam issue (see https://github.com/ocaml/opam/issues/1572) but we can take care of that later. 

@diml : I added documentation and the code is ready to be looked at.

There is annoying point [here](https://github.com/ocsigen/lwt/blob/0ebf3b1b9e44cd0073075b42178bb51685c590a5/_oasis#L332) but there is no clean solution with oasis.

cc @whitequark
@diml queued for next release, it will be behind an AlphaFeature however

@whitequark indeed. Can you please fill 2 bug reports against OASIS (so that you can know when it will be solved).

1. Allow to replace $foo in NativeOpt/ByteOpt
2. Followup on feature request 802 (link in @diml message), to be able to install executable with ocamlfind, it will be behind a feature but I think this is a good idea

Hopefully I can queue all this for the next release, but I am really busy nowaday so I can give you a precise ETA.