Clean up Matcher

[SamStudio8]

• Splits (096d491)
• Orderbook and Matcher should not duplicate work
• Matcher emits Trade with explicit changes (closed orders, split orders)
• Bring some sanity to the price calculation (4d8107c)
• Push business back to domain logic (f87b894)
• Potentially can test get_execution_price instead of entire match_orderbook stack for T7 suite