MAINT: Change signature of create_transaction.

- Change the expected type for order information from the string
  of the order id to an `Order` object, so that it matches the same
  abstraction level as passing in an event.
- Change the order (not to be confused with the parameter named `order`)
  of the parameters so that they go from left to right in order of
  static -> dynamic, i.e. the parameters most likely to change within
  each invoration are the amount and price, with amount more likely
  to change than price.