Implement interactive signing

[marsella]

Closes #467

This PR adds an implementation of interactive signing.

A couple discussion topics:

• I feel like there's an argument for having the signing info be in a type that's either "partial input" or "SignParticipant", instead of the current setup where the participant is optional and we have to clone the partial input to use it (but we never use it again after that). Thoughts on making a bespoke little enum for that?
• The input method on ProtocolParticipant wasn't being used in any protocol-participant-generic methods, so I removed it as a method on that trait. I did this because it doesn't make sense for this implementation to keep a separate copy of the private auxinfo and keygen material around in the InteractiveSignParticipant; better to just pass it to the internal PresignParticipant and wash our hands of it. However, that makes it hard to implement the input() method, so I removed it.
• The Input type here needs a proper constructor. I was thinking of doing that in a separate PR just because this one is already big (I haven't written that issue yet).

[marsella]

@gatoWololo I added said enum; there's a fairly ugly update method that I'd love your thoughts on. Updating a field in a mutable reference in-place is really awkward; I did it with a lot of mem::swaps

[gatoWololo]

@marsella Yeah, that's a hard one. Check out my branch: here I think it is slightly cleaner!

[marsella]

Thanks @gatoWololo, I like your version much better. This is now ready for re-review.