Update tengo vendor and load the stdlib. Fixes #789 (#792)

vendor/github.com/d5/tengo/objects/importable.go

@@ -0,0 +1,7 @@
+package objects
+
+// Importable interface represents importable module instance.
+type Importable interface {
+	// Import should return either an Object or module source code ([]byte).
+	Import(moduleName string) (interface{}, error)
+}