Kaiso

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Start creating labels and constraints for types with unique attributes. This
begins the migration away from legacy indexes towards using labels.

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Bugfix: Ensure all class attributes are set correctly in migration helpers.

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Dropped support for Neo4j 1.9 Cypher in favour of 2.0.

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Migration helpers enhancement to allow the `type_registry` to be previewed when
changing bases for a type.

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Add type coercion and validation. Attribute values for builtin types (e.g. int)
are now coerced before inserting into the db, and for types that use more
permissive primitive types in the db (e.g. uuids stored as strings), values are
also validated to be correctly coercible back when retrieving from the db.

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Bugfix: Manager `get` method raises ValueError if given attr=None.