Update the PPX to use the 4.08 ASTs

[aantron]

• Update constraints in lwt_ppx.opam.

[Drup]

Does it actually need updates ? omp should take care of everything.

[aantron]

This is what I get with the current lwt_ppx...

Source:

let (let*) x f = f x

let () =
  let* p =
    let%lwt () = Lwt_io.printl "Hello world!" in
    Lwt.return ()
  in

  Lwt_main.run p

$ ocamlfind opt -version 
4.08.0
$ ocamlfind opt -linkpkg -package lwt_ppx foo.ml
Fatal error: exception File "foo.ml", line 4, characters 2-6: let operators are not supported before OCaml 4.08
File "foo.ml", line 1:
Error: Error while running external preprocessor

$ dune exec ocamlc -- -version
4.08.0
$ dune build foo.exe
         ppx foo.pp.ml (exit 2)
(cd _build/default && .ppx/95c43a5e8eeb6d851e1995d1bca87765/ppx.exe -o foo.pp.ml --impl foo.ml --dump-ast)
Fatal error: exception File "foo.ml", line 4, characters 2-6: let operators are not supported before OCaml 4.08

I guess that's raised here.

I think we are supposed to keep up with the latest AST.

[aantron]

The commit does fix the errors shown above.