Update hash functions security estimates

[Al-Kindi-0]

We should either update the security estimate of our (algebraic) hash functions or add an argument for the current one.

[Al-Kindi-0]

Given the proof in the last paragraph of Appendix C in the updated version of the xHash paper the following can be closed.

[bobbinth]

Should me make an explicit reference to appendix C somewhere in the code? Or you think what we have is sufficient.

[Al-Kindi-0]

I can make a small PR with an explicit reference.

[bobbinth]

Closed by #320.