Additional bit of precision lost with curve change

[cpacia]

With the change in curves the pack_bits gadget is now dropping the first three bits instead of just the first two as was the case with the prior curves.

This affects the sha256 coprocessor. It's just a one liner change, I can submit a PR if you want but I don't know if there are other areas of lurk that might be affected by this.

[arthurpaulino]

Sure, you can go for the minimal PR and then we evaluate the need of subsequent PRs afterwards.

Thanks for opening the issue!