Gocept-reference

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- Moved InstrumentedSet to use BTree data structures for better performance.

- Added `update` method to InstrumentedSet.

- Updated documentation.

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- Added a `set` implementation for referencing collections of objects.

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- Extended the API for `IReferenceTarget.is_referenced` to allow specifying
whether to query for references recursively or only on a specific object.
By default the query is recursive.

- Fixed bug in the event handler for enforcing ensured constraints: referenced
objects could be deleted if they were deleted together with a parent
location.

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Initial release.