ENH: Add a convenience property for open amount of a blotter.Order

Found that when implementing different slippage model, needed the
open amount in several places/functions, having the open amount
as a property of an order should help reduce the need for passing that
around and maintaining the value separately.
BUG: Fix early exit from fixed slippage when a order amount is zero.

The order loop should continue, effectively dropping/skipping the zero
amount order, not return early and erroneously skip possible valid
orders.