OrderType with MarketOrder support

OrderType.scala

object OrderType {

  def all(): PartialFunction[Order, OrderType] = {

    // Unmodified code omitted
    // ...

    case self@MarketOrder(_, _, _) => new OrderType {
      def bookDisplay: String = "MO"

      def price: PriceLevel = MarketPrice

      def crossesAt(price: Double): Boolean = true

      def decreasedBy(qty: Double): MarketOrder = self.copy(qty = self.qty - qty)
    }
  }
}