Optimize table length computation with hinting.

10x faster on loop with t[#t+1] = x idiom. Also used by table.insert.

src/lj_tab.h

@@ -69,5 +69,8 @@ LJ_FUNC TValue *lj_tab_set(lua_State *L, GCtab *t, cTValue *key);
 
 LJ_FUNCA int lj_tab_next(lua_State *L, GCtab *t, TValue *key);
 LJ_FUNCA MSize LJ_FASTCALL lj_tab_len(GCtab *t);
+#if LJ_HASJIT
+LJ_FUNC MSize LJ_FASTCALL lj_tab_len_hint(GCtab *t, size_t hint);
+#endif
 
 #endif