Square root on BW6 can be optimized

[kobigurk]

It seems like the square root implementation in Zexe is Tonelli-Shanks, and since q = 3 mod 4 it can be optimized. This affects, for example, proof deserialization or random group element generation for testing.

cc @jon-chuang

[jon-chuang]

How much better is this?

[kobigurk]

Not sure tbh, haven't checked.