ACSets should use OrderedSets for their parts

[olynch]

We want to be able to control the order of iteration for parts in an ACSet. In order to do this, the parts should be a Map[Ob, OrderedSet[Part]], where OrderedSet[A] is a class implementing Set[A] that is built out of a Map[A, Int] and Seq[A].

[olynch]

This is fixed in the cleanup branch.