Introduce PropertyDescriptor as an EntryType

[evomimic]

This issue depends upon Issue #39

• Scaffold PropertyDescriptor as EntryType (as a replacement for PropertyDescriptorUsage)
• Comment out use of Composites
• Eliminate "dedicated" value descriptors
• Update test fixtures and test functions for the new design
• Drop PropertyDescriptorMap, PropertyDescriptorUsage, DescriptorSharing structs

[evomimic]

Several challenges have arisen for this issue.

Challenge: SmartLinks Not Available in Descriptors Repo

The updated UML Class Diagram for PropertyDescriptors included SmartLinks (which are not available in this zome).
Resolution: I just scaffolded regular links in lieu of SmartLinks

Challenge: Inverse Link to 2 Different Types

I never really resolved how to do the inverse link from PropertyDescriptor back to EITHER HolonDescriptor OR Composite. Composite is just a flavor of ValueDescriptor and so would require a "property_of" link from PropertyDescriptor back to ValueDescriptor, in addition to the "value_type" link from PropertyDescriptor to ValueDescriptor. The scaffold tool doesn't support "named" links, so it's problematic how to distinguish them without adding more custom code for link creation and navigation.
Resolution: For now I'm just going to ignore Composite. Whether or not Composite could be dropped is an open issue I was going to evaluate after implementing RelationshipDescriptors... but the life of storing Descriptors in this zome vs. the Holons zome is unknown. Just gonna punt on all this for now.

Challenge: Current Testing Strategy Inadequate for Relationships

To make it easy to assert equality between expected and actual test data, we need to create the exact test data in the fixture that we expect to be returned in during the test. But links can't be created in fixtures. How should we handled this?

Resolution: Leverage 'Keys' -- Issue #45

Challenge: Dedicated Value Descriptors

The current design included quite a bit of logic that allowed ValueDescriptors to either be shared (i.e., stored as independent entries) or Dedicated (just stored within their owning usage/PropertyDescriptor). Should this complexity be brought into PropertyDescriptor re-factor?

Resolution: No. All ValueDescriptors will be Shared.