`jacobi` should return `Option<isize>` instead of `isize`

[survived]

jacobi validates input arguments, and if they are invalid, it returns 0 in release build, or panics in debug build. We should rather have jacobi that returns None when provided arguments are invalid. It will help to catch unexpected behavior if it happens.

https://github.com/dfns-labs/paillier-zk/blob/5db04c4a6e36b52656c2ccd9bb9b5dc0fbd66289/src/common/sqrt.rs#L63-L72

[survived]

Actually, we may want jacobi to return Result. It would return:

• Err(Error::InvalidInput) when args are invalid
• Err(Error::Unreachable) when unreachable! branch is reached.