feat(fs): add recursive mkdir, whole-file IO and mtime reads

Path joining, read, write, is_dir and mtime, used by every later stage that
touches disk.

lib/fs.ml

@@ -0,0 +1,29 @@
+let rec mkdir_p dir =
+  if dir <> "" && dir <> "." && dir <> "/" && not (Sys.file_exists dir) then begin
+    mkdir_p (Filename.dirname dir);
+    try Unix.mkdir dir 0o755 with Unix.Unix_error (Unix.EEXIST, _, _) -> ()
+  end
+
+let is_dir p = try Sys.is_directory p with Sys_error _ -> false
+
+let read path =
+  let ic = open_in_bin path in
+  let n = in_channel_length ic in
+  let s = really_input_string ic n in
+  close_in ic;
+  s
+
+let write path s =
+  mkdir_p (Filename.dirname path);
+  let oc = open_out_bin path in
+  output_string oc s;
+  close_out oc
+
+let rec rm_rf p =
+  if is_dir p then begin
+    Array.iter (fun e -> rm_rf (Filename.concat p e)) (Sys.readdir p);
+    try Unix.rmdir p with Unix.Unix_error _ -> ()
+  end
+  else if Sys.file_exists p then try Sys.remove p with Sys_error _ -> ()
+
+let mtime p = try Some (Unix.stat p).Unix.st_mtime with Unix.Unix_error _ -> None