new type 'StackValue' for stack elements

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

llex.c

@@ -1,5 +1,5 @@
 /*
-** $Id: llex.c,v 2.96 2016/05/02 14:02:12 roberto Exp roberto $
+** $Id: llex.c,v 2.97 2017/06/09 16:48:44 roberto Exp roberto $
 ** Lexical Analyzer
 ** See Copyright Notice in lua.h
 */
@@ -129,7 +129,7 @@ TString *luaX_newstring (LexState *ls, const char *str, size_t l) {
   TValue *o;  /* entry for 'str' */
   TString *ts = luaS_newlstr(L, str, l);  /* create new string */
   setsvalue2s(L, L->top++, ts);  /* temporarily anchor it in stack */
-  o = luaH_set(L, ls->h, L->top - 1);
+  o = luaH_set(L, ls->h, s2v(L->top - 1));
   if (ttisnil(o)) {  /* not in use yet? */
     /* boolean value does not need GC barrier;
        table is not a metatable, so it does not need to invalidate cache */