[BIP] Normalized transaction IDs

In an email exchange on October 21, 2015, Gregory Maxwell expressed his concern regarding the deployment of uniform segregated witness (SW) as it would add another identification to every unspent transaction output set and incur additional costs. However, he had been unable to come up with a better solution. Luke Jr. proposed an encoding method that could be used to deploy SW without a disruptive flag day. This method defined a new scriptPubkey type similar to P2SH and required the scriptsig for this scriptPubkey to be empty during the soft-fork. Signatures were not covered under txid but carried alongside and then committed to in blocks in a separate hashtree. The approach used in elements alpha did not disrupt the normal relay handling process, whereas this approach would require a different hash or a separate mechanism for witness transport.