rename & refactor protocols, add WatchedModel type, update/extend TripleGraph impl

- rename PTripleAccess -> PModelUpdate
- rename PModelUpdateHooks -> PModelWatch
- add WatchedModel wrapper w/ pre/post commit hooks (only :add so far)
- add PModel stubs for WatchedModel & TripleGraph
- add `is-leaf?` to PNode & TripleNode
- add `Triple`-typecast to TripleGraph/add-triple
- add `model-size` impl for TripleGraph