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

Closes #1878.

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

examples/web_api.rs

@@ -98,7 +98,7 @@ async fn response_examples(
     }
 }
 
-#[hyper::rt::main]
+#[tokio::main]
 async fn main() -> Result<()> {
     pretty_env_logger::init();