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-reactor/src/lib.rs

@@ -58,7 +58,6 @@ use std::task::Waker;
 use std::time::{Duration, Instant};
 use std::{fmt, usize};
 use tokio_executor::park::{Park, Unpark};
-use tokio_executor::Enter;
 use tokio_sync::task::AtomicWaker;
 
 /// The core reactor, or event loop.
@@ -162,9 +161,9 @@ fn _assert_kinds() {
 /// # Panics
 ///
 /// This function panics if there already is a default reactor set.
-pub fn with_default<F, R>(handle: &Handle, enter: &mut Enter, f: F) -> R
+pub fn with_default<F, R>(handle: &Handle, f: F) -> R
 where
-    F: FnOnce(&mut Enter) -> R,
+    F: FnOnce() -> R,
 {
     // Ensure that the executor is removed from the thread-local context
     // when leaving the scope. This handles cases that involve panicking.
@@ -203,7 +202,7 @@ where
             *current = Some(handle.clone());
         }
 
-        f(enter)
+        f()
     })
 }