[BIP] Normalized transaction IDs

In an email exchange between Christian Decker and Luke Dashjr on October 21, 2015, the issue of malleability in Bitcoin transactions was discussed. Dashjr raised concerns that closing malleability with current methods would not be worth the cost. He suggested specifying flags upfront in the transaction to allow for fully malleability-proof wallets. Decker argued that the remaining venues for malleability were the use of sighash flags that are not SIGHASH_ALL. Signer malleability, where signers can change input order, inputs themselves, add/remove outputs, etc., was also mentioned as a notable concern. However, Dashjr posited that removing signatures from the transaction ID would prevent the signer from changing the transaction ID. The only unique property of the logical transaction is the scriptPubKey/address.