New implementation for to-be-closed variables

To-be-closed variables are linked in their own list, embedded into the
stack elements. (Due to alignment, this information does not change
the size of the stack elements in most architectures.)  This new list
does not produce garbage and avoids memory errors when creating tbc
variables.

ltests.c

@@ -446,6 +446,7 @@ static void checkstack (global_State *g, lua_State *L1) {
   for (uv = L1->openupval; uv != NULL; uv = uv->u.open.next)
     assert(upisopen(uv));  /* must be open */
   assert(L1->top <= L1->stack_last);
+  assert(L1->tbclist <= L1->top);
   for (ci = L1->ci; ci != NULL; ci = ci->previous) {
     assert(ci->top <= L1->stack_last);
     assert(lua_checkpc(ci));