Iterate over vararg

[Dirk Laurie <dirk.laurie@...>]

The task is to write an iterator factory 'args' so that

   for k,v in args(...) do --[[whatever]] end

does the same as

   for k=1,select('#',...) do local v = select(k,...) --[[whatever]] end

It is quite easy by using a table, of course:

~~~~
args = function(...)
  local t = table.pack(...)
  local n = t.n
  local i = 0
  return function()
    i=i+1
    if i<=n then return i,t[i] end
  end
end
~~~~

It seems to be quite hard to do, even in the C API, without creating a
table. In fact, I have so far not succeeded.