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
You can access the variables in setup.data with `BaseEnvLight.var_get` (it is already used in `myocamlbuild.ml`).  You can use the `is_native` variable to guess whether oasis will build the `.byte` or `.native`.