Cleanup privacy on data structures

[Tko1]

Structures like PersistentList (and especially ones like PersistentVector, which will internally change) should not have public internals, and should be interacted with through their public interface; we have already had one potential bug where the cached count was used in PersistentList in an impossible scenario (ie, we were matching on the case of PersistentList(_,_,count = 0) , which would actually never happen, as an empty list would be expressed as PersistentList::Empty)

Off top of my head, need to:

1. Make counting always read with len
2. Use cons to attached a head to a tail
3. Convert PersistentVector into Vector explicitly, rather than doing so by cloning its internal vals Vector