Add stable marshal for container package

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

v2/netmap/marshal.go

@@ -0,0 +1,11 @@
+package netmap
+
+func (p *PlacementPolicy) StableMarshal(buf []byte) ([]byte, error) {
+	// todo: implement me
+	return nil, nil
+}
+
+func (p *PlacementPolicy) StableSize() (size int) {
+	// todo: implement me
+	return 0
+}