Pr6451 fix

This is a fix for http://caml.inria.fr/mantis/view.php?id=6451 .

This consists of 3 kinds of fixes:

* untypeast.ml and pprintast.ml did not generate correct OCaml code
* Some items were stored in the reversed order.
* Some longidents were used by mistake.

This is a simplified version of my branch https://github.com/camlspotter/ocaml/tree/untypeast-4.02 which has succeeded to bootstrap a compiler with parsing => typing => untyping => retyping => code gen phase.