Scalar::sqrt unimplemented

[tarcieri]

The Field trait implementation has a todo!() stub for Field::sqrt in the following crates:

• k256: implement Scalar::sqrt #400
• p256: implement Scalar::sqrt #392

[Pratyush]

FWIW we have generic impls of these in algebra: https://github.com/scipr-lab/zexe/blob/38e792fffc78c0bfc4d780ee21257c8cb70a2e2a/algebra-core/src/fields/arithmetic.rs#L247

[str4d]

We have generic constant-time sqrt implementations in ff_derive for p = 3 mod 4 and p = 1 mod 16: https://github.com/zkcrypto/ff/blob/dcdc2ad3ae1ff50ea689f76bbb667d575cf2a3ff/ff_derive/src/lib.rs#L500-L577