Free bit 7 of GC 'marked' field

Tables were using this bit to indicate their array sizes were real
('isrealasize'), but this bit can be useful for tests. Instead, they
can use bit 7 of their 'flag' field for that purpose. (There are only
six fast-access metamethods.) This 'flag' field only exists in tables,
so this use does not affect other types.

lobject.h

@@ -704,9 +704,9 @@ typedef union Node {
 */
 
 #define BITRAS		(1 << 7)
-#define isrealasize(t)		(!((t)->marked & BITRAS))
-#define setrealasize(t)		((t)->marked &= cast_byte(~BITRAS))
-#define setnorealasize(t)	((t)->marked |= BITRAS)
+#define isrealasize(t)		(!((t)->flags & BITRAS))
+#define setrealasize(t)		((t)->flags &= cast_byte(~BITRAS))
+#define setnorealasize(t)	((t)->flags |= BITRAS)
 
 
 typedef struct Table {