Split tokio-threadpool lib.rs into files (#233)

* Builder -> src/builder.rs
* Callback -> src/callback.rs
* Config -> src/config.rs
* Futures2Wake -> src/futures2_wake.rs
* Inner -> src/inner.rs
* Notifier-> src/notifier.rs
* Sender -> src/sender.rs
* Shutdown -> src/shutdown.rs
* ShutdownTask -> src/shutdown_task.rs
* SleepStack -> src/sleep_stack.rs
* State -> src/state.rs
* ThreadPool -> src/thread_pool.rs
* Worker -> src/worker.rs
* WorkerEntry -> src/worker_entry.rs
* WorkerState -> src/worker_state.rs

tokio-threadpool/src/callback.rs

@@ -0,0 +1,29 @@
+use worker::Worker;
+
+use std::fmt;
+use std::sync::Arc;
+
+use tokio_executor::Enter;
+
+#[derive(Clone)]
+pub(crate) struct Callback {
+    f: Arc<Fn(&Worker, &mut Enter) + Send + Sync>,
+}
+
+impl Callback {
+    pub fn new<F>(f: F) -> Self
+        where F: Fn(&Worker, &mut Enter) + Send + Sync + 'static
+    {
+        Callback { f: Arc::new(f) }
+    }
+
+    pub fn call(&self, worker: &Worker, enter: &mut Enter) {
+        (self.f)(worker, enter)
+    }
+}
+
+impl fmt::Debug for Callback {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        write!(fmt, "Fn")
+    }
+}