Fix for Mantis 6221.

http://caml.inria.fr/mantis/view.php?id=6221

Test case:

    $ cat field.ml
    <:ctyp< a:b >>

Before:

    $ camlp4of field.ml
    File "field.ml", line 1, characters 10-11:
    While expanding quotation "ctyp" in a position of "str_item":
      Parse error: "->" expected after [ctyp level star] (in [ctyp])

After:

    $ camlp4of field.ml
    Ast.TyCol (_loc, (Ast.TyId (_loc, (Ast.IdLid (_loc, "a")))),
      (Ast.TyId (_loc, (Ast.IdLid (_loc, "b")))))
Merge pull request #40 from yallop/mantis-6221

Fix for Mantis 6221.