Added InstanceIdentifier

src/Contracts/InstanceIdentifier.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace Gloudemans\Shoppingcart\Contracts;
+
+interface InstanceIdentifier
+{
+    /**
+     * Get the unique identifier to load the Cart from
+     *
+     * @return int|string
+     */
+    public function getInstanceIdentifier($options = null);
+
+    /**
+     * Get the unique identifier to load the Cart from
+     *
+     * @return int|string
+     */
+    public function getInstanceGlobalDiscount($options = null);
+}