{sign|verify}message replacement

The email exchange between Kalle and Karl Johan Alm discusses a possible replacement for message signing tools that are currently broken for all but the legacy 1xx addresses. The proposed approach is a script-based system with a new structure called SignatureProof, which is a simple scriptSig & witnessProgram container that can be serialized and passed out from/into the signer/verifier. There are two RPC commands: sign, which generates a signature proof for the specified address using the same method that would be used to spend coins sent to that address, and verify, which deserializes and executes the proof using a custom signature checker whose sighash is derived from the specified address. Feedback on the proposal is welcome. However, Kalle notes that a soft fork can make old nodes accept invalid message signatures as valid, so he withdrew his previous proposal (BIP120). Kalle also mentions that Segwit was not "detectable" in this way, meaning that it was not easily detectable by verifiers.