[BIP] Normalized transaction IDs

Similar to the previous version, this proposal also uses a hard fork and only computes the non-malleable parts of a transaction by stripping its signatures before hashing it. For a Coinbase transaction, the normalized transaction ID is equal to the transaction ID (n-txid = txid). However, for non-coinbase transactions, the sigScripts are replaced with empty strings and the txids in TxIns are replaced with n-txid for parents. The second step of this process is recursive starting from the coinbases. Therefore, the rule is that txids are what they would have been if n-txids had been used right from the start.