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.