No more to-be-closed functions

To-be-closed variables must contain objects with '__toclose'
metamethods (or nil). Functions were removed for several reasons:

* Functions interact badly with sandboxes. If a sandbox raises
an error to interrupt a script, a to-be-closed function still
can hijack control and continue running arbitrary sandboxed code.

* Functions interact badly with coroutines. If a coroutine yields
and is never resumed again, its to-be-closed functions will never
run. To-be-closed objects, on the other hand, will still be closed,
provided they have appropriate finalizers.

* If you really need a function, it is easy to create a dummy
object to run that function in its '__toclose' metamethod.

This comit also adds closing of variables in case of panic.

testes/goto.lua

@@ -258,7 +258,7 @@ do
   ::L2:: goto L3
 
   ::L1:: do
-    local *toclose a = function () X = true end
+    local *toclose a = setmetatable({}, {__close = function () X = true end})
     assert(X == nil)
     if a then goto L2 end   -- jumping back out of scope of 'a'
   end