Legendre Symbol

[SteinerMatthias]

Implemented the legendre symbol for BLS12-381 base and scalar fields.

[ebfull]

I had a couple comments.

We don't expose Fp in the crate so there's no need to implement this for Fp since we don't need it internally.

Also, you can just call sqrt and if it returns a valid CtOption you'll know if it's square. Do you have a particular need to more efficiently check whether it's a square outside of the context of computing square roots?

[huitseeker]

@ebfull the classical example would be the Shallue-van de Woestijne map to the curve, I think, but I don't know if this is very relevant for BLS12-381.