Zones domains (sparse_dbm and split_dbm): implement narrowing, widening w/thresholds, strict inequalities, and disequations

[caballa]

• widening with thresholds
• narrowing
• strict inequalities for integers
• disequations (similar precision as intervals)

From @caballa on October 15, 2016 23:33

Copied from original issue: caballa/crab#7