feat(swaps): recalculate partial swap amounts

[ImmanuelSegol]

closes #550

[sangaman]

@ImmanuelSegol Would you mind resolving the conflicts before I review? I know a decent amount was changed so it would be helpful to see what the changes are actually going to look like.

[sangaman]

This is missing changes to OrderBook.handleSwapRequest to accept a swap request for a reduced amount if the full amount is not available.

[sangaman]

This still needs the logic to accept the reduced quantity in the first place, but I'm thinking I can just add that in with a separate PR as it wouldn't hurt to merge this part on its own.

[sangaman]

@ImmanuelSegol Needs a rebase too, but I think let's just make these minor changes and then merge before the next release tomorrow.

[sangaman]

I'm thinking that we'll also need changes to the matching logic to try to rematch any part of a swap request that wasn't accepted, I'll try to handle that as well in the next PR.