Kaiso

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Minor: Expose `for_db` (default: False) via `Manager.serialize` (was previously
only available in the lower-level `TypeRegistry.object_to_dict`). See docs for
`Manager.serialize` for details.

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Add `Manager.has_code_defined_attribute` to determine whether an attribute was
defined on a type in code.

Minor: Removed unnecessary `Manager.is_registered_type` and
`Manager.get_registered_types` methods.

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Add `Manager.change_instance_type` to change the type of existing instances.

Minor: Start reorganising the test suite to have more smaller test modules.

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`Manager.query` (and `Manager._execute`) now return generators of results
instead of being generators themselves. This way, calling `query` triggers
the execution, rather than requiring e.g. `next(manager.query(...), None)`.

Minor: Also fixed the connection tests so they no longer need to be first.

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Explicit type creation: Types are no longer lazily created when instance.save
is called. If the type isn't already persisted, `TypeNotPersistedError` will be
raised.

This enables more aggressive caching of the type registry.