Provide a deterministic alternative to HashMap

[elidupree]

We could implement a deterministic HashMap type (i.e. one where the iteration order depends only on the elements contained). For instance, it could use linear hashing and have each bucket contain a sorted vector (or B-tree) of elements.

We might want to make groups more inherent the TimeSteward. Without special features, storing even a deterministic HashMap in the TimeSteward costs O(n) operations per event that makes a single insertion or deletion. Reducing that back to O(1) would be desirable. On the other hand, storing large amounts of data in a single field is discouraged, so we might not want to spend extra effort to support doing that. And if we do support it, it may also still be useful to create a deterministic HashMap type for use DURING single events.