set: add Singleton

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

set.go

···
 	return !exists
 }
 
+func Singleton[T comparable](item T) Set[T] {
+	n := New[T]()
+	_ = n.Insert(item)
+	return n
+}
+
 func (s *Set[T]) Remove(item T) bool {
 	_, exists := s.data[item]
 	if exists {

set_test.go

···
 	}
 }
 
+func TestPropertySingleonLen(t *testing.T) {
+	f := func(item int) bool {
+		single := Singleton(item)
+		return single.Len() == 1
+	}
+
+	if err := quick.Check(f, nil); err != nil {
+		t.Error(err)
+	}
+}
+
 func TestPropertyInsertIdempotent(t *testing.T) {
 	f := func(s Set[int], item int) bool {
 		clone := s.Clone()