Migrate to cairo-sha256

[tarrencev]

We currently use a fully onchain sha256 hashing algorithm to hashing the signed payload. We should migrate to https://github.com/cartridge-gg/cairo-sha256 once the hints are whitelisted, which provides an ~order of magnitude reduction in sha256 gas costs.

[piniom]

Hi @tarrencev, is this still applicable to our cairo1 code?

[tarrencev]

@ThePinion we do still want to do this i think. IIRC, the current sha256 implementation doesn't parallelize the rounds by packing them into the felt252