Code Review and constraint assumptions review

[enricobottazzi]

For example, this is something I'm not sure about

• https://github.com/enricobottazzi/zk-fhe/blob/master/src/chips/poly_operations.rs#L283
• https://github.com/enricobottazzi/zk-fhe/blob/master/examples/bfv.rs#L254
• https://github.com/enricobottazzi/zk-fhe/blob/master/examples/bfv.rs#L433