compute set difference using a backtracking algorithm

compute set difference using a backtracking algorithm

In particular, treat all basic maps in the minuend separately
and only subtract parts of the subtrahend that actually overlap
with the minuend.  Moreover, only use non-redundant constraints
when performing this subtraction.

Using only non-redundant constraints in overlapping parts
avoids the minuend being split in several parts unnecessarily.
Treating each basic map separately should help speed up
isl_map_is_subset in a subsequent commit.