Add feature flag to hide PRG.

[simon-friedberger]

The motivation for this PR is to remove the dependencies on the crates aes, ctr and cmac so a user of the crate can rely on an existing crypto library. This should be dable by implementing analogues of PrgAes128 and SeedStreamAes128 and constructing, e.g. Prio3Aes128Count = Prio3<Count<Field64>, PrgAes128Alt, 16>;

This would fix the remainder of #241.

[simon-friedberger]

I've created a little copy & paste PoC to see if this actually works: https://github.com/simon-friedberger/priouser

It did require a lot of copy pasting of things for testing. Does anybody have an idea how that could be improved?

[cjpatton]

Apart from deciding what to call the new feature, this is looking good. Just a few minor comments.

[tgeoghegan]

I've created a little copy & paste PoC to see if this actually works: https://github.com/simon-friedberger/priouser

It did require a lot of copy pasting of things for testing. Does anybody have an idea how that could be improved?

Are you referring to struct TPrio3<M> and struct TPrio3Prep? I think we could take a change to expose items like vdaf::{run_vdaf, run_vdaf_prepare} or parts of mod prio3_test under feature test-vector (or test-util if you rename it) to make it easier to develop VDAFs or specific bits of VDAFs outside of crate prio. I think that could land in a subsequent PR, though.

[simon-friedberger]

Yes, I think exposing prio3_test.rs under test-util would probably be helpful.

[cjpatton]

LGTM, I'm ready to approve. First, I think it would be useful to add some context to the commit message. Plus, we should edit the top comment of this PR to point to the issue.

[tgeoghegan]

Looks great, thank you for this change @simon-friedberger!