JJ: Description from the destination commit:

~ syntax stuff

JJ: Description from source commit:
~~

unikernel/syntax.ml

@@ -0,0 +1,22 @@
+let ( let* ) = Lwt.bind
+let ( let+ ) x f = Lwt.map f x
+let ( let*? ) = Lwt_result.bind
+let ( let+? ) x f = Lwt_result.map f x
+
+let list_get_ok l =
+  let err = ref None in
+  try
+    Ok
+      (List.map
+         (function
+           | Error _e as e ->
+               err := Some e;
+               raise Exit
+           | Ok v -> v)
+         l)
+  with Exit -> ( match !err with None -> assert false | Some v -> v)
+
+let lwt_list_get_ok l =
+  let open Lwt.Syntax in
+  let+ l = l in
+  list_get_ok l