Add LJ_FR2 mode: Two-slot frame info.

src/lib_string.c

@@ -58,7 +58,7 @@ LJLIB_ASM(string_byte)		LJLIB_REC(string_range 0)
   lj_state_checkstack(L, (MSize)n);
   p = (const unsigned char *)strdata(s) + start;
   for (i = 0; i < n; i++)
-    setintV(L->base + i-1, p[i]);
+    setintV(L->base + i-1-LJ_FR2, p[i]);
   return FFH_RES(n);
 }
 
@@ -72,7 +72,7 @@ LJLIB_ASM(string_char)		LJLIB_REC(.)
       lj_err_arg(L, i, LJ_ERR_BADVAL);
     buf[i-1] = (char)k;
   }
-  setstrV(L, L->base-1, lj_str_new(L, buf, (size_t)nargs));
+  setstrV(L, L->base-1-LJ_FR2, lj_str_new(L, buf, (size_t)nargs));
   return FFH_RES(1);
 }