rt: Introduce a self-describing box representation and functions to create and free them

src/rt/rust_task.h

@@ -10,6 +10,8 @@
 #include "context.h"
 #include "rust_obstack.h"
 
+struct rust_box;
+
 struct stk_seg {
     unsigned int valgrind_id;
     uintptr_t limit;
@@ -57,7 +59,7 @@ rust_task : public kernel_owned<rust_task>, rust_cond
     context ctx;
     stk_seg *stk;
     uintptr_t runtime_sp;      // Runtime sp while task running.
-    void *gc_alloc_chain;      // Linked list of GC allocations.
+    rust_box *gc_alloc_chain;      // Linked list of GC allocations.
     rust_scheduler *sched;
     rust_crate_cache *cache;