When a new Object is created it can have a `base`, i.e. another
object that is already committed to the store, which is then used
to initialize the tree of the new object. That is, the contents
of the new Object will be based on the contents of the existing.
The initialization of an Object with its base (if any) was done
by the ObjectStore. Move all of that logic inside `Object`:
The Object will store its base, which `Object.init` will use to
initialize itself. Additionally, if `Object.path` is accessed
`init` is being called as well to make sure it is properly
initialized, i.e. the tree initialized with the base content.
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