Add support for indexes to Collections

Implement other indexes as well as the API to create and destroy them.

TODO
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- [x] Add support for hash indexes
- [ ] Add error messages on index constraint violations
- [ ] Add support for geo indexes
- [ ] Add support for skip list indexes
- [ ] Add support for full text indexes
- [ ] Update README.md
This spec contains superfluous expectations, such as checking if the `save` method will not save a record if it invalid. Additionally you could just set the `unique_attribute` manually. Doing so would save you some more expectations.