Provide alternate event emission schema

[alnoki]

Per @qdrs:

• New order placements have a NewOrderEvent
• Order updates (size or price changes) generate an OrderUpdateEvent
• Order cancellations generate an OrderCancelEvent
• When an order is matched:
  • An OrderUpdateEvent to show size changes
  • A FillEvent including a field for how much remaining size is on order

[alnoki]

Per @bogberry consider integrating with #146

[Bogberry]

Per @bogberry consider integrating with #146

I’m not sure what you mean by "integrate." These are two entirely different proposals and you can do either without the other (or, of course, both).