[BIP] Normalized transaction IDs

On October 21, 2015, Gregory Maxwell expressed his sadness over the difficulty in deploying uniform segregated witeness and the added cost of adding another ID to every UTXO set. He had been trying for a long time to find a solution but was unsuccessful. However, Luke came up with a solution that would allow for the deployment of SW without a disruptive flag day using a new P2SH-like scriptPubkey type. This approach requires the scriptsig for the scriptPubkey to be empty during the soft-fork.The signatures are not covered under txid but are carried alongside and then committed to in blocks in a separate hashtree. The only disadvantage to this approach is that it disrupts the normal relay handling process, as relay transporting extradata needs to use a different hash that includes the witness or have a separate mechanism for witness transport.