new type 'StackValue' for stack elements

(we may want to put extra info there in the future)

lcode.c

@@ -1,5 +1,5 @@
 /*
-** $Id: lcode.c,v 2.119 2017/05/18 19:44:19 roberto Exp roberto $
+** $Id: lcode.c,v 2.120 2017/06/27 11:35:31 roberto Exp roberto $
 ** Code generator for Lua
 ** See Copyright Notice in lua.h
 */
@@ -1079,7 +1079,7 @@ static int constfolding (FuncState *fs, int op, expdesc *e1,
   TValue v1, v2, res;
   if (!tonumeral(e1, &v1) || !tonumeral(e2, &v2) || !validop(op, &v1, &v2))
     return 0;  /* non-numeric operands or not safe to fold */
-  luaO_arith(fs->ls->L, op, &v1, &v2, &res);  /* does operation */
+  luaO_rawarith(fs->ls->L, op, &v1, &v2, &res);  /* does operation */
   if (ttisinteger(&res)) {
     e1->k = VKINT;
     e1->u.ival = ivalue(&res);