New syntax for to-be-closed variables

The new syntax is <local *toclose x = f()>. The mark '*' allows other
attributes to be added later without the need of new keywords; it
also allows better error messages.  The API function was also renamed
('lua_tobeclosed' -> 'lua_toclose').

testes/goto.lua

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