{sign|verify}message replacement

A proposal for a new structure called SignatureProof, which is a simple scriptSig & witnessProgram container that can be serialized, has been introduced to replace the broken message signing tools for all but the legacy 1xx addresses. The approach was suggested by Pieter Wuille and does not require much effort for implementation in Bitcoin Core. The new structure is passed out from/into the signer/verifier. The system includes RPC commands such as "sign" and "verify." The "sign" command generates a signature proof using the same method that would be used to spend coins sent to the specified address. On the other hand, the "verify" command deserializes and executes the proof using a custom signature checker whose sighash is derived from the specified address. The scriptPubKey is derived directly from the same address. If verify returns true, the check succeeds; otherwise, it returns false. Feedback is welcome, according to Kalle's proposal. It is suggested that VerifyScript with a new signature checker class can be used, and if sign is true, then the sighash is derived from the specified address, otherwise, it uses sha256d(message).