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/builder.rs

@@ -15,7 +15,6 @@ use std::fmt;
 use std::sync::Arc;
 use std::time::Duration;
 use tokio_executor::park::Park;
-use tokio_executor::Enter;
 
 /// Builds a thread pool with custom configuration values.
 ///
@@ -261,7 +260,7 @@ impl Builder {
     /// use tokio_threadpool::Builder;
     ///
     /// let thread_pool = Builder::new()
-    ///     .around_worker(|worker, _| {
+    ///     .around_worker(|worker| {
     ///         println!("worker is starting up");
     ///         worker.run();
     ///         println!("worker is shutting down");
@@ -272,7 +271,7 @@ impl Builder {
     /// [`Worker::run`]: struct.Worker.html#method.run
     pub fn around_worker<F>(&mut self, f: F) -> &mut Self
     where
-        F: Fn(&Worker, &mut Enter) + Send + Sync + 'static,
+        F: Fn(&Worker) + Send + Sync + 'static,
     {
         self.config.around_worker = Some(Callback::new(f));
         self