Change the order of arguments in ring-Pedersen commitments

[fjarri]

This applies to RPParamsMod.commit* methods. In the paper the commitment is written as s^x t^y where x is the secret, and y is the randomizer, but the commit methods take (randomizer, secret), which may be confusing for someone reading the ZK proofs code.

Also if we change the order, it will be more consistent with the Ciphertext constructor, which has secret first and randomizer second in the argument list.