Multi-secret same signer DLEQ proofs

[burdges]

I designed the VRF to be useful for DLEQ proofs more generally. Right now, we only support DLEQ proofs with merged signing by one private key though, but a DLEQ proof tool should probably do merged signing with multiple private keys known to one signer.

In fact, I initially designed the DLEQ proof tool for merged signing with multiple private keys, but removed it due to complexity concerns. It's worth adding back though eventually.

Anything about multi-signer VRFs comes under #5 for now, btw.

[burdges]

We've so far never discovered a need for this, so I'll close this in favor of #71