(nitro-specific) convert claim and transfer to operate at the single asset level

[geoknee]

[NEW] What is this issue for
We want to integrate the exit format into nitro-protocol. The current contract API for the latter allows for "per asset" transactions.

The pure functions in this repo currently loop over all assets. This is bad because

• we want to allow for finer-grained payouts
• it doesn't make sense to specify a single guaranteeIndex for all assets

[Defined] How will we solve it

Assume that we have already drilled into a given asset for both channels. In fact, write (or port) pure functions that operate per-asset called computeNewAllocation and computeNewAllocationsWithGuarantee.

[Development]
[] This ticket has clearly outlined the expected outcome, and I can complete this ticket with no additional refinement

Additional context Add any other context or screenshots about the feature request here: notion doc, github discussion or ticket