Bytesparse

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* Refactored current implementation as the ``inplace`` sub-module.
* Added abstract base classes and base types into the ``base`` sub-module.
* Removed experimental backup feature.
* Added dedicated methods to backup/restore mutated state.
* Fixed some write/insert bugs.
* Fixed some trim/bound bugs.
* Methods sorted by name.
* Removed useless functions.

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* Using explicit factory methods instead of constructor arguments.
* Added block collapsing helper function.
* Minor fixes.
* Improved test suite.

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* Cython implementation moved to its own ``cbytesparse`` Python package.
* Remote testing moved to GitHub Actions.

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* First release on PyPI.