Simpler Pedersen Swap

[jonasnick]

@LLFourn came up with a much simpler protocol for buying the opening of a Pedersen Commitment than what we currently have: https://github.com/LLFourn/buying-pedersen-commitment/blob/master/main.pdf. Instead of complicated range proof recovery he uses a simple DL equivalence proof. The existing protocol should be removed and replaced with a link to his writeup and some context or a markdown port.