Fix fuzzing empty list items

[mgeisler]

While playing some more with the fuzzer, I found that the normalize function was buggy: it would turn

Start(List(None))
  Start(Item)
  End(Item)
  Start(Item)
  End(Item)
End(List(false))

into

Start(List(None))
  Start(Item)
    Start(Paragraph)
    End(Item)
    Start(Item)
      Start(Paragraph)
      End(Item)
    End(List(false))

by mistake.

This PR moves the logic to a library (inside the fuzzing crate) so we can test this and perhaps reuse it across different fuzzers.

[Martin1887]

Thanks!, I will try to review it this week.

[mgeisler]

Thanks!, I will try to review it this week.

No stress, this is a fun side project for me! 🙂