[Feature Request] implement apairs method in go to complement pairs/ipairs

[terefang]

some custom lua runtimes have the apairs method to complement pairs/ipairs

what apairs basically does it skips over the integer keys and only returns the real hashtable keys

a lua-only implementation is here: https://gist.github.com/jirutka/89eb96b7469a7bfea020

i did some experimenting and found the following go function sufficient

func apairs(t *rt.Thread, c *rt.GoCont) (rt.Cont, error) {
	if err := c.Check1Arg(); err != nil {
		return nil, err
	}
	next := c.Next()
	_arg := c.Arg(0)
	if _tab, _ok := _arg.TryTable(); _ok {
		var _k rt.Value = rt.NilValue
		var _v rt.Value = rt.NilValue
		var _lk rt.Value = rt.NilValue

		for _ok {
			_k, _v, _ok = _tab.Next(_k)
			if _k.IsNil() ||  _k.TypeName() == "string" {
				break
			}
			_lk = _k
		}
		t.Push1(next, rt.FunctionValue(_next))
		t.Push1(next, _arg)
		t.Push1(next, _lk)
		return next, nil
	}
	return nil, errors.New("not a table")
}

thanks in advance