Group similar methods using traits

[fjarri]

Consider RPParamsMod that has three methods commit(), commit_wide() and commit_xwide() which do the same thing, but with different types of integers as arguments. Instead, we can have a Commit trait with different associated types. The same applies e.g. for Ciphertext constructors.

[fjarri]

Doesn't seem to be possible within the Rust type system since e.g. PaillierParams::Uint and PaillierParams::WideUint may be the same type.