Feature request: improve support for Tokio's local runtime and single-threaded servers

I've got a simpler web server that is best run using a single-threaded runtime (it's a local server), and about the only thing standing between me and using `Rc` for some of my app state internals is the fact they aren't `!Send`, and `HttpServer::new` requires its closure to be `Send + Sync`.

Here's what I want:

- A single-threaded local-only variant of `HttpServer::new` that uses only one worker and uses `tokio::task::spawn_local` to spawn whatever helpers it may need.
- An `Rc`-backed version of `web::Data` and `web::ThinData`.

I may have missed other `Send`-requiring types as well - those would also be included in this.