Revert "rt: Remove lock_held_by_current_thread"

Adds back the ability to make assertions about locks, but only under the
--enable-debug configuration

This reverts commit b247de6458.

Conflicts:

	src/rt/rust_sched_loop.cpp

src/rt/rust_sched_loop.cpp

@@ -108,6 +108,8 @@ rust_sched_loop::number_of_live_tasks() {
  */
 void
 rust_sched_loop::reap_dead_tasks() {
+    lock.must_have_lock();
+
     if (dead_task == NULL) {
         return;
     }