We're close to merging DLEQ proofs for Cashu

This was a loooooong journey as you can see it's the oldest open PR in the repo :) We've come up with 2 different schemes that could've replaced this DLEQ proof but none of them is so significantly better that they could replace it.

In short: the DLEQ proof is additional data that is attached to the ecash token that allows a receiver to verify the mint's signature given the mint's public key without having to talk to the mint

It sounds trivial as for most signature schemes, this is easily possible without adding other stuff (you can just verify the signature, that's it).

Why do we need this? It's a necessary building block for our evil plan of implementing ecash zaps on nostr and other cool things like offline receive.