dev: make modexp great again

[Eikix]

as seen together teccak will repair modexp and make kakarot great again ♥️

Tasks

Beta Give feedback

1. check modexp specification
2. ignore kakarot modexp implementation and start over
3. use new circuits?? on starknet to reimplement modexp soundly
4. decide whether to implement modexp not for arbitrary length uints, but rather up to 256 bits

[tekkac]

Ahoy sir, as per seen offline, I am interested in this endeavor. I would oblige if I could get the chance to be assigned.

[enitrat]

@tekkac as a first task, a modexp version that works with inputs of size < 256 bits would be nice. I think it should be doable with Garaga.