Function `EncryptWithR` lets to specify randomness `r` explicitely. For
some ZKPs based on the Paillier scheme, it's required to use the same
`r` for encryption and constructing ZKP. It wasn't possible previously,
because `r` was generated internally and never returned.
I did not want to modify `Encrypt` to return `r` because it would break
existing code using this API, as well as, I want to make sure person
using the specific `r` and using it somewhere else really knows what
he's doing.
Jakub Nowakowski