feat: add support for iteration on rar archive

[MunifTanjim]

iter, err := rardecode.OpenIter(firstVolumeFilename, rardecode.FileSystem(fs), rardecode.IterHeadersOnly)
if err != nil {
	panic(err)
}
defer iter.Close()

for iter.Next() {
	header := iter.Header()
	println(header.Name)
}
if err := iter.Err(); err != nil {
	panic(err)
}