Support Payment Proofs

Since the sender address of an output is just an ephemeral key, and the receiver address is a one-time key generated from a stealth address, the sender needs to be able to prove to an arbiter that a payment was made in case of disputes. This will involve proving:

1. The output existed - This can be done with a merkle proof that the output was once in the output PMMR.
2. Amount - This can be done by proving the blinding factor (using a signature), and showing that amount*H + blind*G = Output commitment. Alternatively, we could just reveal the secret key used to encrypt the value & blind.
3. Receiver address belongs to recipient
4. Sender address belongs to sender
5. Blinding factor is stored in encrypted data - If we just reveal the secret key used to encrypt the value & blind, we get this for free.