Embed values of an arbitrary OCaml type 'a in an Elisp value. The values are not transformed, so this can be used to preserve state in Emacs. More precisely, the following returns true:

  let var = Var.create symbol (Caml_embed.create_type type_id) in
  Current_buffer.set_value var v;
  phys_equal v (Current_buffer.value_exn var)