[BIP] Normalized transaction IDs

Even if a canonical ordering is instated, there may not be much that can be done about it, as the signer could simply sign a completely different transaction spending the same coins elsewhere. Normalized transaction IDs can help prevent modification in multi-signer scenarios and immediate follow-up transactions by the same signer. However, Luke Dashjr argues that third-party manipulation is not much more of a problem than signing-party manipulation and that solving the former without solving the latter is not worth the high cost.