Enhancement: Add privately scoped mono-typed keys as an alternative to URef.

- Offers contract writers an alternative to URefs to keep track of local "stuff". Stretch feature. Not to be considered until after Uref rework is complete (separate Epic)
- Right now: anything put into global state has to use a URef. COULD add support for a non-sharable, context bound, key. Similar to the old notion of local-key (which created a different set of challenges and was removed in favor of dictionaries). Dictionaries use a base hash made of an arbitrary URef addr + hash of a unique identifying sub-key to generate a hash (not contract or account hash).
- New solution (trait) would be similar to a dictionary (but different): Allow a URef to have a single stored value to use as a seed. Thus contract package versions would use the same seed to maintain access to their local keys.