Add Server::respond

[yoshuawuyts]

This adds Server::respond, replacing most of the functionality provided by the http-service trait. This allows passing arbitrary requests into the server, and always get responses back out. The goal is that we remove the http-service functionality in a future PR entirely, and replace it with this method instead.

What this enables us to do is to construct requests using Surf, and then pass them into Tide without a problem:

let mut app = tide::new();
app.at("/").get(|_| async move { Ok("hello world") });

let req = surf::get("https://business.soup");
let res: surf::Response = app.respond(req).await?;

assert_eq!(res.status(), 200);

Screenshots

[yoshuawuyts]

Tests are failing locally. Closing in favor of #503