Add examples/fs showing how to walk a directory tree

Author: talex5

examples/fs/dune

@@ -0,0 +1,3 @@
+(executable
+  (name main)
+  (libraries eio_main))

examples/fs/main.ml

@@ -0,0 +1,21 @@
+(* Walks the directory tree rooted at the current directory,
+   displaying all directory names (skipping hidden directories and `_build`). *)
+
+open Eio.Std
+
+let ( / ) = Eio.Path.( / )
+
+let rec scan t =
+  match Eio.Path.kind ~follow:false t with
+  | `Directory ->
+    traceln "Visiting %a" Eio.Path.pp t;
+    Eio.Path.read_dir t |> List.iter (function
+      | "_build" -> ()
+      | item when String.starts_with ~prefix:"." item -> ()
+      | item -> scan (t / item)
+    )
+  | _ -> ()
+
+let () =
+  Eio_main.run @@ fun env ->
+  scan (Eio.Stdenv.cwd env)