Implement a lock-free work-stealing deque

This adds an implementation of the Chase-Lev work-stealing deque to libstd
under std::rt::deque. I've been unable to break the implementation of the deque
itself, and it's not super highly optimized just yet (everything uses a SeqCst
memory ordering).

The major snag in implementing the chase-lev deque is that the buffers used to
store data internally cannot get deallocated back to the OS. In the meantime, a
shared buffer pool (synchronized by a normal mutex) is used to
deallocate/allocate buffers from. This is done in hope of not overcommitting too
much memory. It is in theory possible to eventually free the buffers, but one
must be very careful in doing so.

I was unable to get some good numbers from src/test/bench tests (I don't think
many of them are slamming the work queue that much), but I was able to get some
good numbers from one of my own tests. In a recent rewrite of select::select(),
I found that my implementation was incredibly slow due to contention on the
shared work queue. Upon switching to the parallel deque, I saw the contention
drop to 0 and the runtime go from 1.6s to 0.9s with the most amount of time
spent in libuv awakening the schedulers (plus allocations).

Closes rust-lang#4877

src/librustuv/net.rs

@@ -1076,21 +1076,22 @@ mod test {
         use std::rt::task::Task;
         use std::rt::task::UnwindResult;
         use std::rt::thread::Thread;
-        use std::rt::work_queue::WorkQueue;
+        use std::rt::deque::BufferPool;
         use std::unstable::run_in_bare_thread;
         use uvio::UvEventLoop;
 
         do run_in_bare_thread {
             let sleepers = SleeperList::new();
-            let work_queue1 = WorkQueue::new();
-            let work_queue2 = WorkQueue::new();
-            let queues = ~[work_queue1.clone(), work_queue2.clone()];
+            let mut pool = BufferPool::init();
+            let (worker1, stealer1) = pool.deque();
+            let (worker2, stealer2) = pool.deque();
+            let queues = ~[stealer1, stealer2];
 
             let loop1 = ~UvEventLoop::new() as ~EventLoop;
-            let mut sched1 = ~Scheduler::new(loop1, work_queue1, queues.clone(),
+            let mut sched1 = ~Scheduler::new(loop1, worker1, queues.clone(),
                                              sleepers.clone());
             let loop2 = ~UvEventLoop::new() as ~EventLoop;
-            let mut sched2 = ~Scheduler::new(loop2, work_queue2, queues.clone(),
+            let mut sched2 = ~Scheduler::new(loop2, worker2, queues.clone(),
                                              sleepers.clone());
 
             let handle1 = Cell::new(sched1.make_handle());