* Remove private Graphcache exports

In theory these undocumented APIs could be used to write a custom
normalized cache based on Graphcache's internal logic. However, we
forgot to expose `initDataState` anyway and additionally these APIs
were not actually in use by anyone. So for now we'll remove them.

* Simplify getFragments utility in traversal helpers

* Reduce iteration cost of SelectionIterator

- Replace object with just an iterate function
- Reduce cost of iteration by adding recursive iterators

* Add changeset