Use a nominal datatype for CamlinternalFormat.format6
    
    This should make the type-checking of formats simpler and more robust:
    instead of trying to find a pair as previously, we can now use the
    path of the format6 type directly.
    
    A nice side-effect of the change is that the internal definition of
    formats (as a pair) is not printed in error messages anymore.
    Because format6 is in fact defined in the CamlinternalFormatBasics
    submodule of Pervasives, and has an alias at the toplevel of
    Pervasives, error messages still expand the definition:
    
    > Error: This expression has type
    >          ('a, 'b, 'c, 'd, 'd, 'a) format6 =
    >            ('a, 'b, 'c, 'd, 'd, 'a) CamlinternalFormatBasics.format6
    >        but an expression was expected of type ...
    
    Passing the option `-short-paths` does avoid this expansion and
    returns exactly the same error message as 4.01:
    
    > Error: This expression has type ('a, 'b, 'c, 'd, 'd, 'a) format6
    >        but an expression was expected of type ...
    
    (To get this error message without -short-paths, one would need to
    define format6 directly in Pervasives; but this type is mutually
    recursive with several GADT types that we don't want to add in the
    Pervasives namespace unqualified. This is why I'll keep the alias
    for now.)
    
    Conflicts:
    	typing/typecore.ml
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@14866 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02