Consolidate key share interfaces

[marsella]

Right now, the creation of KeySharePublic and KeySharePrivate happens on-the-fly in a function called new_keyshare, and the fields on those types are public and mutable. It would be better to have private, read-only fields with a validated constructor so that we are more confident that those types are always correct.

Update: the fields are now private, as of #371.

Since the public key share is derived from the private, it makes sense to either derive them together in a single constructor or require that the public type be derived from an existing private instance, as for paillier::{EncryptionKey, DecryptionKey}.

Related issue: #52. PiSch public and private inputs are just key shares.

• [x] Make methods to generate public / private key shares -- done in #371
• [x] Make all fields on key shares private, add getter functions if necessary -- done in #371
• [ ] Remove the new_keyshare function and replace with a direct call to the constructors

[marsella]

This is mostly a duplicate of #207.