[BIP] Normalized transaction IDs

In an email exchange, Jorge Timón and Christian Decker discuss the use of the term "malleability" in relation to Bitcoin transactions. Timón argues that there is a terminology confusion between conflicting spends of the same outputs (unconfirmed double-spends) and signature malleability, which Segregated Witnesses solves. Decker agrees and clarifies that his main goal is to eliminate the risk of detaching transactions in off-blockchain protocols that rely on a number of transactions being chained, hence solving signature malleability might be the correct term. They both agree that Segregated Witnesses would be a perfect solution, but a good migration plan for Bitcoin needs to be found.