isl_basic_map_gist: replace by new version based on tableaus

isl_basic_map_gist: replace by new version based on tableaus

The new version should be a little bit more efficient because
it only constructs a single tableau.

More importantly, the new version keeps better track of the original
constraints and never introduces an inequality constraint in the result
that did not appear in the input.
This is especially important in the presence of divs.
Throught variable compression, the original version would sometimes
translate constraints that did not involve any divs into constraints
that did involve divs.