change isl_vertex inspectors to return proper isl objects

change isl_vertex inspectors to return proper isl objects

When these functions were introduced back in 5f0f628 (add parametric
vertex enumeration, Mon Jun 7 14:27:13 2010 +0200), isl did not
have any support for multiple affine expressions, which were only
added in 3d0da01 (add isl_multi_aff, Thu Sep 1 11:49:19 2011 +0200).
We therefore exposed the internal representation of the vertices
as rational sets, but rational sets are not really supported by isl and
required special attention by the user.
Now that we have isl_multi_aff objects, we can change the interface
to return proper isl objects.  This change breaks users, but there
should be fairly few users given the awkward nature of the original
interface.  The new result type should be easier to handle than
the original.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>