Clarify which keyword usage is in which keyword scope

[erights]

With a lot of our uses of various keyword records, it is often not clear, or easy to forget, which keyword namespace these keywords are relative to. Here in particular, the from and the to are wrt two different seats that have two different keyword namespaces. However, having fromLosses default to toGains is only correct if the keyword namespaces agree. This will be easy to miss under maintenance --- that the absence of this argument means that the keyword namespaces of these two seats cannot evolve independently.

Altogether, I'm surprised by how big calcNewAllocations is, and how big its caller, trade, is.

_Originally posted by @erights in #1414 (comment)

[katelynsills]

Let's investigate using a helper to compare the keywords of the seats in swap. Or somehow declare that a contract has consistent keywords. Consider assertions of using the same keywords in different seats.

[katelynsills]

Perhaps shouldn't be able to use the defaulting in trade without some assertion of the keywords of the seats being the same.