Added some casts for 32-bit machines

When both 'int' and 'l_obj' have 32 bits, an unsigned int needs a
cast to be assigned to 'l_obj'. (As long as 'l_obj' can count the
total memory used by the system, these casts should be safe.)

lgc.c

@@ -624,7 +624,7 @@ static l_mem traversetable (global_State *g, Table *h) {
         linkgclist(h, g->allweak);  /* must clear collected entries */
       break;
   }
-  return 1 + 2*sizenode(h) + h->asize;
+  return cast(l_mem, 1 + 2*sizenode(h) + h->asize);
 }