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

Closes #1878.

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

examples/proxy.rs

@@ -5,7 +5,7 @@ use hyper::{Client, Error, Server};
 use hyper::service::{make_service_fn, service_fn};
 use std::net::SocketAddr;
 
-#[hyper::rt::main]
+#[tokio::main]
 async fn main() {
     pretty_env_logger::init();