Add mli for bytes

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/stdune/bytes.mli

@@ -0,0 +1,9 @@
+include module type of StdLabels.Bytes with type t = StdLabels.Bytes.t
+
+val blit_string
+  :  src:string
+  -> src_pos:int
+  -> dst:t
+  -> dst_pos:int
+  -> len:int
+  -> unit