set: introduce set difference

Signed-off-by: oppiliappan <[e-mail]>

set.go

···
 	}
 	return result
 }
+
+// Difference returns a new set with elements in s but not in other.
+func (s Set[T]) Difference(other Set[T]) Set[T] {
+	result := New[T]()
+	for k := range s {
+		if _, ok := other[k]; !ok {
+			result.Add(k)
+		}
+	}
+	return result
+}