Make Paillier keys opaque (by having them generate proofs)

[Lúcás Meier]

Instead of exposing all the fields in Paillier keys, we can instead expose necessary methods to generate proofs.

Advantages:

• Simpler and more robust API
• Easier to convert to safenum internally (awaiting broader usage)

You could also expose accessors to the fields, but this has the same API complexity.