Add validation of auxinfo input

[marsella]

After #242 is complete, auxinfo will take some input. It should validate that the input is correct. This can happen in the AuxInfoParticipant::new method -- see presign for an example.

• Add a check that there's exactly one input key share for each participant. Write a test.
• Think about whether there's anything else reasonable to check.

This might be rendered irrelevant by #439, which adds some validation to keygen output.