feat: Marshal untagged variable T as rust Option<T> (#6)

In some case, Option wrapped values are not in a struct.
Here introduce a wrapper to do the rust style Option wrapper to wrap
the target value.

bcs/encode.go

@@ -262,6 +262,32 @@ func Marshal(v any) ([]byte, error) {
 	return b.Bytes(), nil
 }
 
+type Option[T any] struct {
+	Some T
+	None *struct{}
+}
+
+func (p *Option[T]) MarshalBCS() ([]byte, error) {
+	if p.None != nil {
+		return []byte{0}, nil
+	}
+	b, err := Marshal(p.Some)
+	return append([]byte{1}, b...), err
+}
+
+func (p *Option[T]) UnmarshalBCS(r io.Reader) (int, error) {
+	buf := new(bytes.Buffer)
+	io.Copy(buf, r)
+	tmp := buf.Bytes()
+	if len(tmp) == 1 {
+		var emptyStruct struct{}
+		p.None = &emptyStruct
+		return 1, nil
+	}
+	b := tmp[1:]
+	return Unmarshal(b, &p.Some)
+}
+
 // MustMarshal [Marshal] v, and panics if error.
 func MustMarshal(v any) []byte {
 	result, err := Marshal(v)

bcs/encode_test.go

@@ -137,3 +137,46 @@ func TestMarshal_optional(t *testing.T) {
 		t.Errorf("want: %v\ngot:  %v\n", optionalSetExpected, optionalSetBytes)
 	}
 }
+
+func TestMarshal_option(t *testing.T) {
+	t.Run("some", func(t *testing.T) {
+		var p0, p1 bcs.Option[[]byte]
+		input := []byte{0xC0, 0xDE}
+		inputExpected := []byte{1, 2, 0xC0, 0xDE}
+		p0.Some = input
+		b, err := bcs.Marshal(&p0)
+		if err != nil {
+			t.Error(err)
+		}
+		if !sliceEqual(b, inputExpected) {
+			t.Errorf("want: %v\ngot:  %v\n", inputExpected, b)
+		}
+		_, err = bcs.Unmarshal(b, &p1)
+		if err != nil {
+			t.Error(err)
+		}
+		if !sliceEqual(input, p1.Some) {
+			t.Errorf("want: %v\ngot:  %v\n", input, p1.Some)
+		}
+	})
+	t.Run("none", func(t *testing.T) {
+		var p0, p1 bcs.Option[[]byte]
+		inputExpected := []byte{0}
+		var emptyStruct struct{}
+		p0.None = &emptyStruct
+		b, err := bcs.Marshal(&p0)
+		if err != nil {
+			t.Error(err)
+		}
+		if !sliceEqual(b, inputExpected) {
+			t.Errorf("want: %v\ngot:  %v\n", inputExpected, b)
+		}
+		_, err = bcs.Unmarshal(b, &p1)
+		if err != nil {
+			t.Error(err)
+		}
+		if p1.None == nil {
+			t.Errorf("None field should have value")
+		}
+	})
+}