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/rt/rust_builtin.cpp

@@ -850,6 +850,11 @@ unpin_task(rust_task *task) {
     task->unpin();
 }
 
+extern "C" CDECL rust_chan *
+clone_chan(rust_task *task, rust_chan *chan) {
+    return chan->clone(task);
+}
+
 //
 // Local Variables:
 // mode: C++