Add `arith.cmpi` lowering from Iliashenko & Zucca to `--secret-to-bgv`

Implement techniques from ["Faster homomorphic comparison operations for BGV and BFV"](https://petsymposium.org/popets/2021/popets-2021-0046.php) [^1]

Caveat: this approach is only really practical for small plaintext modulus (e.g.,$p = 257$), but that requires advanced CRT splitting into $R_{p^d}$ for $d > 1$, which is not supported by OpenFHE.

[^1]: Iliashenko, I. and Zucca, V. 2021. Faster homomorphic comparison operations for BGV and BFV.
Proceedings on Privacy Enhancing Technologies. 2021, 3 (Jul. 2021), 246–264.