Get rid of Enter for with_default (#1315)

We want executors to enforce that there are never multiple active at the
same time. This is ensured through `Enter`, which will panic if you
attempt to create more than one. However, by requiring you to pass an
`&mut Enter` to `executor::with_default`, we were *also* disallowing
temporarily overriding the current executor.

This patch removes that requirement.

tokio-threadpool/src/worker/mod.rs

@@ -119,11 +119,11 @@ impl Worker {
             let mut sender = Sender { pool };
 
             // Enter an execution context
-            let mut enter = tokio_executor::enter().unwrap();
+            let _enter = tokio_executor::enter().unwrap();
 
-            tokio_executor::with_default(&mut sender, &mut enter, |enter| {
+            tokio_executor::with_default(&mut sender, || {
                 if let Some(ref callback) = self.pool.config.around_worker {
-                    callback.call(self, enter);
+                    callback.call(self);
                 } else {
                     self.run();
                 }