Speedup sqrt on PrimeField when a square root exists.

[ValarDragon]

Description

Applies daira's suggestion from #40 to make the square root algorithm skip computing the legendre symbol. This speeds up the square root when the element is actually a square root, in exchange for slowing down the square root when its not actually a square root.

I think this makes sense as a trade-off, as getting faster point decompression in the honest case is quite useful. And in cases I'm aware of, with an adversary who want to delay another node, you would have made your curve point pass deserialization and fail in other logic later on

cref #40

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[Pratyush]

Perfect, thanks!