token: introduce Sealed::is_valid

To go along with Capability::is_valid

sdk/include/token.h

diff --git a/sdk/include/token.h b/sdk/include/token.h
index c25c04b..00111ec 100644
--- a/sdk/include/token.h
+++ b/sdk/include/token.h
@@ -175,6 +175,13 @@
 	{
 		return reinterpret_cast<T *>(sealedPointer);
 	}
+	/**
+	 * Return the tag of the underlying pointer
+	 */
+	bool is_valid()
+	{
+		return __builtin_cheri_tag_get(get());
+	}
 };
 
 /**