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.

testes/events.lua

@@ -305,6 +305,17 @@ t[Set{1,3,5}] = 1
 assert(t[Set{1,3,5}] == undef)
 
 
+do   -- test invalidating flags
+  local mt = {__eq = true}
+  local a = setmetatable({10}, mt)
+  local b = setmetatable({10}, mt)
+  mt.__eq = nil
+  assert(a ~= b)   -- no metamethod
+  mt.__eq = function (x,y) return x[1] == y[1] end
+  assert(a == b)   -- must use metamethod now
+end
+
+
 if not T then
   (Message or print)('\n >>> testC not active: skipping tests for \z
 userdata <<<\n')