Add support for ImmutableList<T> as the recursive collection

[AArnott]

Fixes #69

[AArnott]

Some of the tests that were modified are assertion 'current' behavior even though that behavior isn't desirable. For example, MovingNodeAroundHierarchy asserts that removing a node in the front of the list causes the next item to report PositionUnderParent as changed (since it moved from index 1 to 0). But this is undesirable, since it is implied by the removal of another element in the list.

Ideally, PositionUnderParent is only reported for nodes that have changed positions when comparing the before and after list with only those contents that are present in both.