[Router] Support `.use()` middleware mounting

[danneu]

val router = Router {
    use(middleware1())
    // ... A routes ...
    use(middleware2())
    // ... B routes ...
    use(middleware3())
}

• middleware1 touches every request.
• middleware2 touches requests that were not handled by any A routes.
• middleware3 touches requests that would otherwise 404 since no routes matched/responded.