More work on word-count.

Updated the MapReduce protocol so that it's correct more often. It's
still not perfect, but the bugs repro less often now.

Also found a race condition in channel sending. The problem is that
send and receive both need to refer to the _unread field in
circular_buffer. For now I just grabbed the port lock to send. We can
probably get around this by using atomics instead.

src/lib/task.rs

@@ -5,6 +5,9 @@ native "rust" mod rustrt {
     fn unsupervise();
     fn pin_task();
     fn unpin_task();
+    fn clone_chan(*rust_chan c) -> *rust_chan;
+
+    type rust_chan;
 }
 
 /**
@@ -44,6 +47,11 @@ fn unpin() {
     rustrt::unpin_task();
 }
 
+fn clone_chan[T](chan[T] c) -> chan[T] {
+    auto cloned = rustrt::clone_chan(unsafe::reinterpret_cast(c));
+    ret unsafe::reinterpret_cast(cloned);
+}
+
 // Local Variables:
 // mode: rust;
 // fill-column: 78;