[#159] sdk: Define object range in object pkg

Define Range type in object package. Replace Range in client package with
the new one.

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

pkg/object/range_test.go

@@ -0,0 +1,25 @@
+package object
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestRange_SetOffset(t *testing.T) {
+	r := NewRange()
+
+	off := uint64(13)
+	r.SetOffset(off)
+
+	require.Equal(t, off, r.GetOffset())
+}
+
+func TestRange_SetLength(t *testing.T) {
+	r := NewRange()
+
+	ln := uint64(7)
+	r.SetLength(ln)
+
+	require.Equal(t, ln, r.GetLength())
+}