threadpool: update to std::future (#1219)

An initial pass at updating `tokio-threadpool` to `std::future`. The
codebase and tests both now run using `std::future` but the wake
mechanism is not ideal. Follow up work will be required to improve on
this.

Refs: #1200

tokio-threadpool/src/waker.rs

@@ -0,0 +1,24 @@
+use crate::pool::Pool;
+use crate::task::Task;
+
+use arc_waker::Wake;
+use std::sync::Arc;
+
+/// Implements the future `Waker` API.
+///
+/// This is how external events are able to signal the task, informing it to try
+/// to poll the future again.
+#[derive(Debug)]
+pub(crate) struct Waker {
+    pub pool: Arc<Pool>,
+    pub task: Arc<Task>,
+}
+
+unsafe impl Send for Waker {}
+unsafe impl Sync for Waker {}
+
+impl Wake for Waker {
+    fn wake_by_ref(me: &Arc<Self>) {
+        Task::schedule(&me.task, &me.pool);
+    }
+}