{sign|verify}message replacement

Luke-Jr, a Bitcoin Core developer, commented on a proposal to replace the message signing tools and suggested a new signature format instead of a new RPC interface. He proposed that the new format should not only support "proof I receive at this address" but also "proof of funds" as a separate feature, which can prove funds for multiple inputs. Additionally, he suggested that the new scheme should avoid disclosing public keys for existing or future UTXOs, but this may not be possible without something MAST-like first. The proposal by Karl Johan Alm suggested adding a new structure called SignatureProof, which is a simple scriptSig and witnessProgram container that can be serialized. Feedback was welcomed for this system.