Harmonize curve25519-signature format with what others are doing
Here's another one that we have to do before the 0.2.7.2-alpha release if we're going to do it at all:
I'm not in love with the differences here, but they seem mostly harmless. Trevor tells me that there's a chance of his that format being standardized. If so, we might as well help it along by doing what they do.
Changing the nonce-generation mechanism is IMO not very important. The change to consider before the next alpha would be the signature format.