subspec: implement Poseidon2 spec with tests

[tcoratger]

No description provided.

[khovratovich]

Using arbitrary constant values whereas the rest of the spec follows the original paper looks odd. Consider pre-generating the constants and storing them in a constant array, or rename the function as non-compliant.

[tcoratger]

Thanks for your comment. I just fixed it and I agree.

Actually, having this super simple way of setting constants without having to generate an arbitrary array allowed me to compare the results with an existing implementation without adding too many lines of code with a hardcoded array because I didn't have any specific values to put in this array other than random ones.

What I propose:

• I've modified the name and description of the existing function to clearly state that it's for generating test values, not something production-ready.
• I'll open a follow-up issue. For BabyBear, we have hardcoded round constants: https://github.com/Plonky3/Plonky3/blob/bd6fb4116132d64c100cd5338567354b8d7dab1f/baby-bear/src/poseidon2.rs#L185-L243 which come from the HorizonLabs implementation (I imagine this set of values provides some advantages in terms of speed) but we don't have the equivalent for KoalaBear. Maybe it could be nice to have such a list for KoalaBear and once we have it, hardcode it inside the spec to remove the function and rely on this efficient round constants.