Remove intermediate Service struct

[yoshuawuyts]

This implements HttpService directly on Server removing the need for the intermediate Service struct. This allows us to remove into_http_service and the server submodule making e.g. nesting apps really easy.

Examples

let mut app = tide::new();
let mut inner_app = tide::new();
inner_app.at("/").get(|_| async { Ok("hello world") });
app.at("/foo").nest(inner_app);
app.listen("localhost:8080").await?;

Future directions

In http-rs/http-types#94 we've introduced the (experimental) Client and Server traits. The hope is that these can replace http-client and http-service including on Tide directly. Because these traits don't carry a connection this would allow us to bypass http_service_mock::make_server and http_mock_service::TestBackend::simulate calls entirely by simply calling app.recv_req instead, making it really easy to test applications:

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

let req = Request::new(Method::Get, "http://localhost:8080");
let res = app.recv_req(req).await?;