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

vendor/github.com/d5/tengo/stdlib/builtin_modules.go

@@ -0,0 +1,14 @@
+package stdlib
+
+import "github.com/d5/tengo/objects"
+
+// BuiltinModules are builtin type standard library modules.
+var BuiltinModules = map[string]map[string]objects.Object{
+	"math":  mathModule,
+	"os":    osModule,
+	"text":  textModule,
+	"times": timesModule,
+	"rand":  randModule,
+	"fmt":   fmtModule,
+	"json":  jsonModule,
+}