Tri-declarative

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* `sort_after()` should produce an error when attempting to sort after non-existant keys

* Tweaked namespace merge in `setdefaults_path`

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* Fix corner case in collect_namespaces where one parameter imply a value and
others imply a namespace.

* Added `setdefaults_path` helper with `__` namespace traversal.

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* Added `assert_kwargs_not_empty` convenience function.

* Improved documentation.

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* When making instances of a class decorated with `declarative` the declared
values are copied (shallow) before being passed to `__init__`.

* Instances will get an own copy of the declared attributes written to their
`__dict__`