specs 0.17.0

Specs is an Entity-Component-System library written in Rust.
Documentation
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## Checklist

* [ ] I've added tests for all code changes and additions (where applicable)
* [ ] I've added a demonstration of the new feature to one or more examples
* [ ] I've updated the book to reflect my changes
* [ ] Usage of new public items is shown in the API docs

## API changes

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