Rename to nim-poseidon2-bn254

[bkomuves]

As we are now dealing with different fields and different hash functions, in particular we have poseidon2 version(s) which are different from this one, I would prefer to rename this to nim-poseidon2-bn254 to signify that this one is over the BN254 curve's scalar field.

I don't want to put all of them into the same package, because this was has a very heavy dependency (constantine), while the other one is self-contained.

Not sure how painful is the renaming process in practice, or how to organize / synchronize it.