Using Lwt with ppx_let instead of ppx_lwt

lwt_ppx_let.ml

module Let_syntax = struct
  let return = Lwt.return
  let (>>=) = Lwt.Infix.(>>=)
  let (>>|) = Lwt.Infix.(>|=)
  module Let_syntax = struct
    let bind m ~f = Lwt.bind m f
  end
end