[#140] sdk: Refactor reference types

Refactor v2-compatible reference types to be wrappers over corresponding
types from v2.

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

pkg/object/id_test.go

@@ -9,7 +9,7 @@ import (
 )
 
 func TestIDV2(t *testing.T) {
-	id := new(ID)
+	id := NewID()
 
 	checksum := [sha256.Size]byte{}
 
@@ -20,8 +20,5 @@ func TestIDV2(t *testing.T) {
 
 	idV2 := id.ToV2()
 
-	id2, err := IDFromV2(idV2)
-	require.NoError(t, err)
-
-	require.Equal(t, id, id2)
+	require.Equal(t, checksum[:], idV2.GetValue())
 }