add isl_pw_*_{take,restore}_space
add isl_pw_*_{take,restore}_space

These make it easier to write functions that decompose
a piecewise expression, such as isl_pw_multi_aff_range_factor_*,
which will be added in an upcoming commit.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl