rt: remove unsafe from shell runtime. (#2333)

Since the original shell runtime was implemented, utilities have been
added to encapsulate `unsafe`. The shell runtime is now able to use
those utilities and not include its own `unsafe` code.

tests-integration/tests/rt_shell.rs

@@ -0,0 +1,32 @@
+#![warn(rust_2018_idioms)]
+#![cfg(feature = "sync")]
+
+use tokio::runtime;
+use tokio::sync::oneshot;
+
+use std::sync::mpsc;
+use std::thread;
+
+#[test]
+fn basic_shell_rt() {
+    let (feed_tx, feed_rx) = mpsc::channel::<oneshot::Sender<()>>();
+
+    let th = thread::spawn(move || {
+        for tx in feed_rx.iter() {
+            tx.send(()).unwrap();
+        }
+    });
+
+    for _ in 0..1_000 {
+        let mut rt = runtime::Builder::new().build().unwrap();
+
+        let (tx, rx) = oneshot::channel();
+
+        feed_tx.send(tx).unwrap();
+
+        rt.block_on(rx).unwrap();
+    }
+
+    drop(feed_tx);
+    th.join().unwrap();
+}