0.0-3 Added are_disjoint_sets, are_intersecting_sets.
      Removed withr dependency.
0.0-2 is_set_equal renamed as are_set_equal.
      is_subset and is_superset gain a "strictly" argument.
      Use of devtools in tests replaced with withr.
0.0-1 Content extracted from assertive 0.3-0, and tidied.
