Use `k256::FieldElement` for `secq256k1::Scalar`

[kayabaNerve]

We're blocked by the lack of PrimeFieldBits, and FromUniformBytes<[u8; 64]> would be preferred (RustCrypto/elliptic-curves#1379 (review)). Additionally, it's failing to have its docs generated so that would be worth fixing...

[kayabaNerve]

Blocked by RustCrypto/elliptic-curves#1703 and RustCrypto/elliptic-curves#531 (comment).