Lots of work on memory tracking and channels.

We're trying to get closer to doing correct move semantics for channel
operations. This involves a lot of cleanup (such as removing the
unused sched parameter from rust_vec constructor) and making
circular_buffer kernel_owned.

Added tagging for memory allocations. This means we give a string tag
to everything we allocate. If we leak something and TRACK_ALLOCATIONS
is enabled, then it's much easier now to tell exactly what is leaking.

src/rt/rust_scheduler.cpp

@@ -290,7 +290,8 @@ rust_scheduler::get_cache() {
 rust_task *
 rust_scheduler::create_task(rust_task *spawner, const char *name) {
     rust_task *task =
-        new (this->kernel) rust_task (this, &newborn_tasks, spawner, name);
+        new (this->kernel, "rust_task")
+        rust_task (this, &newborn_tasks, spawner, name);
     DLOG(this, task, "created task: " PTR ", spawner: %s, name: %s",
                         task, spawner ? spawner->name : "null", name);
     if(spawner) {