Consider Limit Orders as User Orders

[nlordell]

Throughout the code, we assume order_trades == user_trades (in order words, all custom price trades are non-user orders). This is because previously, we only had market and liquidity orders, so the distinction was true.

This PR adds removes order_trades and custom_price_trades methods in favour of new trades (for all trades) and user_trades (for user trades) methods, which IMO helps clarify intent as to which trades we care about at the various call-sites.

With this, we fix the issue where limit orders aren't considered as user orders in some places.

One thing I noticed and am not very fond of is the current Settlement vs SettlementEncoder abstraction. Specifically, I think the way that the current separation is designed contributed to this being an issue in the first place. I plan on submitting a follow-up PR that will refactor this into something that will be (IMO) nicer to use and less error prone.

Test Plan

Existing tests continue to pass. Adjusted some tests to create limit orders to be considered as user orders.

[nlordell]

I just realized we aren't computing surplus properly for limit orders either.

[MartinquaXD]

LGTM. 👍