Add missing_ok to Eio.Path.unlink

[vog]

It would be very helpful if there was a variant of Eio.Path.unlink that doesn't require the file to exist.

One possibility would be an optional missing_ok argument, similar to what we have in rmtree, e.g.:

let unlink ?(missing_ok=false) path =
  match missing_ok with
  | false -> original_unlink path
  | true ->
    try original_unlink path
    with Eio.Io (Eio.Fs.(E (Not_found _)), _) -> ()

The main difference to rmtree is that with this version one cannot accidentally delete whole directory sub trees.

Does that make sense? Would you accept a PR for that?

[talex5]

Seems reasonable to me.

(since unlink already installs an exception handler, you might as well just add another case there)