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 I tried; but the ppx executable is present in tree with the name `ppx/ppx_lwt_ex.byte` and not `ppx_lwt`, so I have no idea how to trick OASIS into installing it with the proper name.

No, there is no better way. One of the reasons I migrated from OASIS now.