Change ordering of fields in ETF encoded events

[Ruttie2006]

Currently ETF encoded gateway events are sent with the d, op, s and t fields in arbitrary order.
In practice, I always encounter the fields in the d, op, s, t order.
Because the type of the d field can only be determined by the op and t fields, decoding of the event requires skipping the d field (possibly transcoding it into a temporary representation), then reading the op and t fields, only to then decode the d field once more with actual type info.
This makes decoding rather wasteful, because you effectively have to decode the event twice.
Ideally the fields would have the d field occur last, with the order of the fields being a guarantee rather than a guess.