Publicize non-zero weierstrass affine variables

[alex-ozdemir]

Description

I've found these to be useful for a recent project.

I also added an EqGadget implementation and a test.

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[alex-ozdemir]

Sorry for the delay; I was out-of-town.

We were working with a prime-order curve and had a large computation that could produce intermediate zeros with only negligible probability.