Per-thread scheduling. Closes #682.

Tasks are spawned on a random thread. Currently they stay there, but
we should add task migration and load balancing in the future. This
should drammatically improve our task performance benchmarks.

src/rt/rust_task.h

@@ -91,6 +91,7 @@ rust_task : public maybe_proxy<rust_task>,
 
     // Indicates that the task ended in failure
     bool failed;
+    bool propagate_failure;
 
     lock_and_signal lock;