Add augment_plus and augment_minus to PrivateKey

[coltfred]

See #94

TODO

• tests showing proper Fr augmentation.

[clintfred]

Did you consider introducing a generic parameter onto PrivateKey to handle the Fp256 vs Fr256. An advantage would be that + and - could be separately defined for PrivateKey<Fp256> vs PrivateKey<Fr256>.
Another potential solution would be to make two separate types.

[coltfred]

@clintfred Previously to hide the underlying FpXXX types from the public api. If we didn't want to do that still we could do what you're suggesting. I would argue that there is limited use of doing that and it just pushes more information onto the caller than they care about.

One could argue that we should remove - and + from the public apis as they have limited value to a caller.

[clintfred]

I agree. I would suggest we remove + and - from PrivateKey until we have a use for it. It seems like it will serve to confuse users, and if we ever needed it again it's simple to add back in.

Unless someone else has a usecase for it that I'm not thinking of...

[clintfred]

LGTM