Support "mounting" subrouters and route-sensitive middleware

[aturon]

Right now, all routes and middleware must be set up at the top-level App. However, it's often useful to be able to nest an existing router (or potentially App at a given route inside another App. Similarly, it should be possible to set up middleware at specific nestings within a router, so that it only applies to paths with a specific prefix.

To sketch, this might look something like:

let mut app = App::new(my_data);
app.at("/foo").nest(|router| {
    router.middleware(some_middlware); // only applies to routes within this nesting
    router.at("bar").get(bar_endpoint); // ultimately routes `/foo/bar` to `bar_endpoint`
})

Let's nail down the API design and then work out a good implementation approach.

[tirr-c]

I came up with an idea, will try when I have enough time (maybe this weekend.)

Basic idea is,

• Make Router::at public
• Move middleware into Router
• App::router(&mut self) will return top level router
• Resource contains either a map of endpoints or a nested router
• Resource::nest<F: FnOnce(&mut Router)>(&mut self, builder: F) will build a new Router and mount it

[J-F-Liu]

I think the middleware design of Koa framework is quite good, can we do something similar in Rust?

app.use(async (ctx, next) => {
  const start = Date.now();
  await next();
  const ms = Date.now() - start;
  ctx.set('X-Response-Time', `${ms}ms`);
});

[tirr-c]

@J-F-Liu Seems like #54 is the thing.