Move back extra outputs at the _end_

  For rapid testing, I originally change the way extra outputs (i.e. the change output) were added to the transaction to match the same convention used by the PAB. That is, the PAB have them by default as the first output of every transaction, and contract traces are built with this assumption in mind.
  However, having them at the beginning is really not handy, since there may be in practice more than one change output! It is thus easier to append them to existing outputs, and have contracts works from a 0-based index instead.