Use gnark directly for Fp and Bandersnatch curve as much as possible & allow code regen for Fr

[jsign]

This PR is a work in progress to use gnark as a direct dependency (as much as possible) instead of using their tool to generate code.

A high-level summary is:

• For our Fp we use gnark.bls12-381.Fr directly, with some extra helper methods. (Similar take to a previous attempt)
• I created a Makefile rule to quickly regenerate our Fr, since there's no package in gnark for that. I regenerated the code with the latest (v0.11.0) gnark version, so we have the latest version for a field of these characteristics. With the new version of gnark, we can do make gen-fr and it will regenerate again.
• Went a bit further and used gnark/ecc/bls12-381/bandersnatch package directly, with some extra helper methods. So, also using gnark for the curve.
• I keep our in our Fp extra methods, our faster square root algorithm. This is important to allow banderwagon to deserialize points faster.
• Generally, I make adjustments here and there to accommodate new APIs. I'll go in more detail in PR comments when opening, but nothing surprising.

This is still a WIP.

[jsign]

Closing in favor of https://github.com/crate-crypto/go-ipa/pull/53. That PR is less ambitious, but let's go in smaller steps.