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

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

@@ -0,0 +1,23 @@
+package objects
+
+// BuiltinModule is an importable module that's written in Go.
+type BuiltinModule struct {
+	Attrs map[string]Object
+}
+
+// Import returns an immutable map for the module.
+func (m *BuiltinModule) Import(moduleName string) (interface{}, error) {
+	return m.AsImmutableMap(moduleName), nil
+}
+
+// AsImmutableMap converts builtin module into an immutable map.
+func (m *BuiltinModule) AsImmutableMap(moduleName string) *ImmutableMap {
+	attrs := make(map[string]Object, len(m.Attrs))
+	for k, v := range m.Attrs {
+		attrs[k] = v.Copy()
+	}
+
+	attrs["__module_name__"] = &String{Value: moduleName}
+
+	return &ImmutableMap{Value: attrs}
+}