add a new simple, builtin requestid middleware (makes use of the Context.SetID/GetID methods too)

Former-commit-id: d46bce7c1964adada01934aa95daf389c141defc

_examples/experimental-handlers/cors/simple/client/main.go

@@ -9,6 +9,6 @@ func main() {
 	})
 
 	// Read index.html comments,
-	// adn then start and navigate to http://localhost:8080.
+	// and then start and navigate to http://localhost:8080.
 	app.Listen(":8080")
 }