rt: Refactor Runtime::block_on to take &self (#2782)

Co-authored-by: Eliza Weisman <[email protected]>

tokio-util/tests/context.rs

@@ -2,11 +2,11 @@
 
 use tokio::runtime::Builder;
 use tokio::time::*;
-use tokio_util::context::HandleExt;
+use tokio_util::context::RuntimeExt;
 
 #[test]
 fn tokio_context_with_another_runtime() {
-    let mut rt1 = Builder::new()
+    let rt1 = Builder::new()
         .threaded_scheduler()
         .core_threads(1)
         // no timer!
@@ -21,8 +21,5 @@ fn tokio_context_with_another_runtime() {
 
     // Without the `HandleExt.wrap()` there would be a panic because there is
     // no timer running, since it would be referencing runtime r1.
-    let _ = rt1.block_on(
-        rt2.handle()
-            .wrap(async move { delay_for(Duration::from_millis(2)).await }),
-    );
+    let _ = rt1.block_on(rt2.wrap(async move { delay_for(Duration::from_millis(2)).await }));
 }