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
All other ppx executables in the ecosystem (mine, Alain's and someone else's) start with `ppx_`.