Add presign tests and testing infrastructure

[marsella]

Right now if we want to test signing we need to run all of presiging first to get a PresignRecord (or, more interestingly, a set of consistent PresignRecords). It would be nice to have a test-only function to quickly simulate the presign protocol without actually running it. There are similar methods for keygen and auxinfo outputs.

To do this, we need to figure out what properties the presign record must satisfy. A record is a tuple (R, k_i, chi_i) such that:

• R is a curve point. It is a jointly-randomly-selected point g^(k^-1) where k is a random integer.
• k_i is a scalar. It's a share of the random integer k
• chi_i is a scalar. It's a share of x * k where x is the ecdsa secret key (distributed among all the parties)

A set of valid presign records should have the following properties, but I'm not completely confident that these are necessary or sufficient. We'll need to generate a key pair to start with (doesn't have to be distributed); denote the public key X and private key x.

• All parties have the same R.

• R = g^((sum k_i)^-1)

• sum(chi_i) = x * (sum k_i) mod q. I'm not quite sure what multiplicative notation this is -- k and x are both just numbers mod q (sampled from F_q) so it might be straight up multiplication

• [x] There's no test for presign in general; to confirm that the above properties are correctly stated, it would be wise to write a test that runs presign correctly and checks the properties once it finishes

• [x] Figure out if there are additional properties to uphold

• [x] Write a test-only function to simulate a set of consistent Presign records (plus the verifying key) for a given # of participants. This can be modeled after the simulate methods on keygen::Output and auxinfo::Output.

[marsella]

Observation while working on this issue: Testing the last condition is kind of awkward because our secret keys are BigNumbers but they're being compared to k256::Scalars. I wonder if we should be using that k256::Scalar type more throughout the library, specifically for every time we generate something in 𝔽_q.