Add top-level printing for OCaml values:

   # let s = ocaml_string_start "abcde";;
   val s : string Ctypes.ocaml = "abcde"
   # s +@ 2;;
   - : (string, [ `OCaml ]) Ctypes.pointer = "abcde" [offset:2]
Add top-level printing for OCaml types:

   # ocaml_string;;
   - : bytes ocaml typ = char*
Introduce a direct-mapping type for `bytes'.