These are used to build the IContainer/IContained classes.

This makes selections of CloneEntityHook simpler, cheaper and more robust.
Drop IClonableAdapter.clone property, not longer useful.

Also add the `structure` method which returns the fully evaluated
graph (of schema objects), useful for testing.

Actual tests are not so specific to "entities" despite most of the code
currently lies there and they are not so unit either.