refactor(rt): remove re-export of tokio::main (#1879)

Closes #1878.

BREAKING CHANGE: Replace all usage of `rt::main` with `tokio::main`.

examples/state.rs

@@ -6,7 +6,7 @@ use std::sync::{Arc, atomic::{AtomicUsize, Ordering}};
 use hyper::{Body, Error, Response, Server};
 use hyper::service::{make_service_fn, service_fn};
 
-#[hyper::rt::main]
+#[tokio::main]
 async fn main() {
     pretty_env_logger::init();