[BIP] Normalized transaction IDs

In an email discussion, Christian Decker expressed interest in Luke's idea of having empty scriptsigs and shipping the signatures in external scripts but noted that the proposal uses on-the-fly normalization because there is no good way of relaying the external scripts. Decker is open to suggestions for a better solution. Changing the network protocol is easy compared to making a permanent increase in UTXO set costs. The issue with malleability is not changing inputs/outputs, but rather that such changes invalidate later spends. To create an anti-malleable wallet, one would need to strictly enforce the no-address-reuse rule on payments received and rely only on the hash of that scriptPubKey+value for the input in subsequent transactions. This way, no matter what inputs or other outputs the transaction paying the address/invoice uses, the subsequent transaction ignores them and remains valid. This is not a mandatory change, but a possible upgrade for wallets to adopt in the future.