add keygen output constructor and tests

[marsella]

Closes #376

This adds into_parts and from_parts methods for keygen output so that we can reconstruct them in products that need to store the output. It also:

• Moves the keygen output to its own module
• Updates the simulate methods for keygen and auxinfo to only produce valid outputs
• Updates the presign input validation to reduce double-checking of things that are already checked in the individual output constructors.

[gatoWololo]

Neat. Looks good to me.

[hridambasu]

Looks good to me so far with Omar's points.