Trade Mechanics should be a Separate Sub/Spec

[nobu-maeda]

Trade Mechanics should be a separate spec and not part of XIP-01. This to emphasize that there can be many Trade Mechanics, and to be clear how much the change in Trade Mechanics can change the order and trade flow.

Trade Mechanic specifications should be clear what the state machine states should be on both the Maker and Taker side, and what event they should be waiting on before moving state.

Last but not least, a Trade Mechanic shall detail out what are the error and failure scenarios, and whether there's any means to retry or handle the errors.