Add hashssdif and hashdif.
Add hashssdif and hashdif.

hashssdif: The size of the difference of a finite set and a subset is the set's size minus the subset's.

hashdif: The size of the difference of a finite set and another set is the first set's size minus that of the intersection of both.

metamath/set.mm,metamath/set.mm,metamath/set.mm