Bw6-761 optimized Miller loop

[mmagician]

Description

Companion to arkworks-rs/algebra#617

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• Wrote unit tests
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[weikengchen]

Just to check: is the pairing equivalent or not? --- latter would be breaking.

[mmagician]

The pairing's equivalent. But you bring up a good point - the parameters for the Miller loop need to be adjusted, so maybe the ec change should be considered as a breaking change?
This would only be breaking if someone had their own implementation of another bw6 curve and was already using arkworks 0.4+. In the end, we should probably stick to best practices though.

[weikengchen]

If the pairing is equivalent, then I think it would be completely okay---we do this, but just leave a changelog entry that the parameters have been changed.

[weikengchen]

^ and in this case, ec change should surely be a breaking change.

[weikengchen]

I believe we can merge this one first and then see if the test in the algebra passes.

[Pratyush]

@mmagician Let's merge this once the parameters are updated.

[mmagician]

I temporarily switched to a branch bw6-optimization from algebra - this should allow us to verify that all tests are indeed passing, and once this PR is merged, we can also merge arkworks-rs/algebra#617, and finally update the patch.