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/test/run-pass/lib-task.rs

@@ -0,0 +1,41 @@
+
+
+// xfail-stage0
+
+use std;
+import std::task;
+
+fn test_sleep() { task::sleep(1000000u); }
+
+fn test_unsupervise() {
+  fn f() {
+    task::unsupervise();
+    fail;
+  }
+  spawn f();
+}
+
+fn test_join() {
+  fn winner() {
+  }
+
+  auto wintask = spawn winner();
+
+  assert task::join(wintask) == task::tr_success;
+
+  fn failer() {
+    task::unsupervise();
+    fail;
+  }
+
+  auto failtask = spawn failer();
+
+  assert task::join(failtask) == task::tr_failure;
+}
+
+fn main() {
+  // FIXME: Why aren't we running this?
+  //test_sleep();
+  test_unsupervise();
+  test_join();
+}