[2.2] get_user and get_user_id methods

Closes #5429

includes/class-wc-order.php

@@ -115,6 +115,24 @@ class WC_Order {
 		return $value;
 	}
 
+	/**
+	 * Gets the user ID associated with the order. Guests are 0.
+	 * @since  2.2
+	 * @return int|false
+	 */
+	public function get_user_id() {
+		return $this->customer_user ? $this->customer_user : 0;
+	}
+
+	/**
+	 * Get the user associated with the order. False for guests.
+	 * @since  2.2
+	 * @return WP_User|false
+	 */
+	public function get_user() {
+		return $user_id = $this->get_user_id() ? get_user_by( 'id', $user_id ) : false;
+	}
+
 	/**
 	 * Check if an order key is valid.
 	 *