[#130] sdk: Define object identifier type

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/object/id_test.go

@@ -0,0 +1,27 @@
+package object
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestIDV2(t *testing.T) {
+	id := new(ID)
+
+	checksum := [sha256.Size]byte{}
+
+	_, err := rand.Read(checksum[:])
+	require.NoError(t, err)
+
+	id.SetSHA256(checksum)
+
+	idV2 := id.ToV2()
+
+	id2, err := IDFromV2(idV2)
+	require.NoError(t, err)
+
+	require.Equal(t, id, id2)
+}