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

Closes #1878.

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

examples/client.rs

@@ -11,7 +11,7 @@ use hyper::Client;
 // A simple type alias so as to DRY.
 type Result<T> = std::result::Result<T, Box<dyn std::error::Error + Send + Sync>>;
 
-#[hyper::rt::main]
+#[tokio::main]
 async fn main() -> Result<()> {
     pretty_env_logger::init();