Improve code quality for PiEncProof

[marsella]

There are a few code quality items missing in the PiEncProof module.

• [x] Add top-level documentation to PiEncProof explaining what statements it's proving.
• [x] Update fields in proof, input, and secret with more meaningful/limited types where appropriate
• [x] Update field names in proof, input and secret to be more descriptive and add single-letter names to field docs
• [x] new functions should take cloned types by value, not reference

[marsella]

If this gets done before #40, add a list of types that should be ZStarN on that issue for easy reference.

[marsella]

Notes for #128. In PiEncProof:

• plaintext_commit and plaintext_mask_commit should be Commitments.
• randomness_response should be MaskedRandomness.

The remaining BigNumbers are actually just integers and can keep that type.