adds module for dleq proofs

[armfazh]

Add DLEQ proofs of knowledge.

[lukevalenta]

I confirmed that the implementation matches https://www.ietf.org/id/draft-irtf-cfrg-voprf-09.html#name-discrete-log-equivalence-pr (and also read https://blog.cloudflare.com/privacy-pass-the-math/ for some additional context).

The code itself LGTM, but there's no documentation. Adding some comments describing how the functions translate to the draft specification would be helpful. I'll leave some comments inline with more detailed suggestions.

[cdrubin]

Please add a link to the paper and page or similar where many of these symbols names come from so that variables like M, Z and Ci, Di have context.

[armfazh]

Please add a link to the paper and page or similar where many of these symbols names come from so that variables like M, Z and Ci, Di have context.

This was covered by documentation comments added by Luke pointing to the draft's URL.