Allow optimisation of Deltas for sub reducers

[mrpmorris]

When a reducer (e.g. School) uses a sub-state reducer (e.g. Students) it might be necessary to transform the Delta state to improve performance.

For example, if the Delta is a List<Student> and we need the sub reducer to check each student in its given state and look up that student in the Delta then it would be better if we could first transform the List<Student> to a Dictionary<int, Student> for quick lookup.