{sign|verify}message replacement

In an email correspondence, Luke Dashjr, a Bitcoin Core developer, suggested that the current message signing feature should support creating proofs of funds as a separate feature. This would be useful since the current message signing only proves the receiving address and not the funds. Dashjr proposed that it should be possible to create a proof for specific unspent transaction outputs (UTXOs), hence "inputs". This way, users could provide a mix of addresses and inputs, and the system would generate a proof that signs the message for each input, taking the scriptPubKey from the address or UTXO data directly on the blockchain.