[BIP] Normalized transaction IDs

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.