Add the ability to increase the number of Feistel Rounds

[bgrieder]

This is a simple PR to allow increasing the number of Feistel rounds. It does not change the default FF1 API (that uses 10 rounds) but offers 2 additional FF1 structures

• FF1fr that takes as a const generic the number of desired Fesitel rounds
• FF1h that set the number of rounds to 18

The value 18, comes from this cryptanalysis paper: https://eprint.iacr.org/2020/1311.pdf, page 18, last paragraph before the acknowledgments