make Diametric::Entity a class

This is not so much a 'request'.  It's more of a pull-question to start a pull-discussion, with code attached for clarity.

As I jumped in to Diametric, I rapidly started thinking of my classes which included `Entity` as *being* Entities. That made me wonder how much work it would be turn Entity into a class which could be inherited from.  

But once that was done, I didn't know how to tell if it was an improvement. 

I wanted to ask the maintainers: what do you see as the relative advantages of APIs built on module inclusion and those built on class inheritance?