Revise the implementation of PartialAssignment

[gfrances]

A PartialAssignment (i.e. partial state) - could be perhaps more efficient with no map involved, just one state plus a bool vector, etc.

[miquelramirez]

Does still bother us? We are using standard maps here, perhaps another kind of tree will work better performance-wise with the kind of access patterns that arise, and we don't have to complicate the implementation.

[gfrances]

Not that it bothers a lot, but perhaps I'll have to look at the performance of it... at some point.

[miquelramirez]

Sure thing :)