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.)

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14868 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
replay trunk@14523: warn on non-principal format6 coercions

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14869 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02