Use Blake2b instead of SHA

[spaceships]

We used SHA because the Blake2b library we use does not support output lengths we need - error at runtime for length 300+.

We could maybe work around it by just using length extension like H(x,1) || H(x,2) || H(x,3). Do you think this is kosher @iquerejeta?

[abakst]

This question may just be an artifact of our old hash_to_curve implementation. Let's return to this issue after #5 to see if we need to resolve it.

[markulf]

Is there a benefit from not using whatever cardano is using? Is cardano calling a rust or a C library? (libsodium)

Ideally hash functions should be easy to replace. Cautionary tale of MD5 being very hard to remove reliably from legacy software.