Introduced task handles.

This is the new way to refer to tasks in rust-land. Currently all they
do is serve as a key to look up the old rust_task structure. Ideally
they won't be ref counted, but baby steps.

src/rt/rust_task.h

@@ -34,7 +34,6 @@ struct gc_alloc {
     }
 };
 
-
 struct
 rust_task : public kernel_owned<rust_task>, rust_cond
 {
@@ -59,6 +58,8 @@ rust_task : public kernel_owned<rust_task>, rust_cond
     size_t gc_alloc_thresh;
     size_t gc_alloc_accum;
 
+    rust_task_id id;
+
     // Keeps track of the last time this task yielded.
     timer yield_timer;