Sleep trait (#665)

This enables customizing executor sleeping strategies.

Currently, it is not possible to run a Tokio reactor and a 
`CurrentThread` executor on the same thread. The executor is
hard coded to use condvars for sleeping and the Tokio reactor
requires calling `epoll_wait` (or equivalent) for blocking the
current thread.

The `Sleep` trait is added to abstract over this sleeping strategy.

Beyond just supporting the Tokio reactor, adding a `Sleep` trait is
useful for integrating any logic that requires hooking into the sleep
strategy (e.g. timers).

`executor::CurrentThread` is then modified to accept a `Sleep` value
that allows specifying a custom sleep strategy.

benches/current_thread_executor.rs

@@ -19,7 +19,7 @@ fn execute_oneshot(b: &mut Bencher) {
     b.iter(move || {
         let cnt = Rc::new(Cell::new(0));
 
-        CurrentThread::block_with_init(|_| {
+        CurrentThread::run(|_| {
             for _ in 0..ITER {
                 let cnt = cnt.clone();
                 CurrentThread::execute(lazy(move || {
@@ -41,7 +41,7 @@ fn execute_yield_many(b: &mut Bencher) {
     b.iter(move || {
         let cnt = Rc::new(Cell::new(0));
 
-        CurrentThread::block_with_init(|_| {
+        CurrentThread::run(|_| {
             for _ in 0..TASKS {
                 let cnt = cnt.clone();
                 let mut rem = YIELDS;
@@ -83,7 +83,7 @@ fn execute_daisy(b: &mut Bencher) {
     b.iter(move || {
         cnt.set(0);
 
-        CurrentThread::block_with_init(|_| {
+        CurrentThread::run(|_| {
             daisy(DEPTH, cnt.clone());
         });