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_task.cpp

@@ -212,6 +212,7 @@ rust_task::must_fail_from_being_killed() {
 
 bool
 rust_task::must_fail_from_being_killed_unlocked() {
+    kill_lock.must_have_lock();
     return killed && !reentered_rust_stack;
 }