Support required attributes

[loganmhb]

Datomic doesn't support requiring an entity to have attributes, but I think this could be a useful feature to help ensure data consistency. Probably the way this would work is to have a db:entityType attribute, and then provide a way to define entity types and declare the set of required attributes.

There might be other type-level assertions that could be useful to add as well. I think it's a good idea not to disallow attributes, however -- that would make the schema too inflexible and difficult to grow. I think there's an analogy here to how clojure.spec works, allowing you to specify required map keys but not disallow additional keys.