Fix Lwt_unix.lstat calling Unix.stat on Win32

[MisterDA]

A nice little bug here ;)

It can become rather nasty with this code (how I found it):

  Lwt_main.run begin
      with_temp_dir @@ fun dir ->
      let link = Filename.(concat dir "link") in
      Unix.symlink ~to_dir:true "C:/Windows" link;
      do_stuff_here ()
    end
  (* If [link] is a directory, Lwt will enter it and try to
     delete its contents *)

A little grep in Lwt tells that only Lwt_io.with_temp_dir uses Lwt_unix.lstat.

[avsm]

Yikes, this is quite a serious bug on Windows. @raphael-proust -- think we should cut a point release with this?

[smorimoto]

Before cutting a point release, we have to resolve the incompatibility with 4.02 introduced by #880 or update the downward version constraint.

[MisterDA]

An opam-grep for Lwt_unix.lstat gives the list of opam packages affected on Windows below. Some packages may be missing if they opened Lwt_unix.lstat. Thanks to @Ngoguey42 for running it.

[Info] Getting the list of all known opam packages..
[Info] Fetching and grepping using grep..
current matches your regexp.-----------------------------------------]  891/3500
current_ansi matches your regexp.------------------------------------]  892/3500
current_docker matches your regexp.----------------------------------]  893/3500
current_examples matches your regexp.--------------------------------]  894/3500
current_git matches your regexp.
current_github matches your regexp.----------------------------------]  896/3500
current_incr matches your regexp.------------------------------------]  897/3500
current_rpc matches your regexp.-------------------------------------]  899/3500
current_slack matches your regexp.-----------------------------------]  900/3500
current_web matches your regexp.
fat-filesystem matches your regexp.----------------------------------] 1144/3500
flow matches your regexp.--------------------------------------------] 1175/3500
lwt matches your regexp.##########-----------------------------------] 1678/3500
lwt_ppx matches your regexp.######-----------------------------------] 1683/3500
lwt_react matches your regexp.####-----------------------------------] 1684/3500
obuilder-spec matches your regexp.######-----------------------------] 2000/3500
obuilder matches your regexp.###########-----------------------------] 2001/3500
opam-monorepo matches your regexp.###########------------------------] 2231/3500
pvem_lwt_unix matches your regexp.###################----------------] 2664/3500
rashell matches your regexp.
release matches your regexp.##########################---------------] 2733/3500
tezt matches your regexp.######################################------] 3222/3500
:heavy_check_mark: [##################################################################] 3500/3500