Implement the `Iterator` trait for the `Tokenizer`

Relates to: https://github.com/torrust/bencode2json/issues/12

The [Tokenizer](https://github.com/torrust/bencode2json/blob/develop/src/tokenizer/mod.rs#L30-L32) has a method `next_token` that could be implemented as an iterator.