Added a library version of spawn. Before long, we can remove the old version.

src/rt/rust_task.h

@@ -109,6 +109,7 @@ rust_task : public kernel_owned<rust_task>, rust_cond
 
     void start(uintptr_t spawnee_fn,
                uintptr_t args);
+    void start();
     void grow(size_t n_frame_bytes);
     bool running();
     bool blocked();