feat: try to optimise gamma_pow_acc in a different way

[iAmMichaelConnor]

I wanted to quickly test the gate improvement of computing gamma_i with poseidon hashing instead of exponentiation.

And then I spend ABSOLUTELY AGES WITH CLAUDE TRYING TO FIGURE OUT HOW TO UPDATE THE REST OF THIS REPO

[iAmMichaelConnor]

OMG IT PASSED TESTS

[iAmMichaelConnor]

Not sure how to actually measure the improvement in gate count... the flamegraph tool hangs when trying to process the checkpoint root circuit

[iAmMichaelConnor]

Won't merge. Not a worthwhile saving. Exponentiating gamma gives 254-bits of security, so we're fine.