PR: Implement support for fast integer digest using "xxhash".

[KelSolaar]

Summary

This PR implements support for fast integer digest using xxhash with the colour.utilities.int_digest definition. If xxhash is not available it falls back to the builtin hash definition.

The main use case is hashing of the ndarray as highlighted in PR #1120.

Preflight

Code Style and Quality

• Unit tests have been implemented and passed.
• Pyright static checking has been run and passed.
• Pre-commit hooks have been run and passed.
• [N/A] New transformations have been added to the Automatic Colour Conversion Graph.
• [N/A] New transformations have been exported to the relevant namespaces, e.g. colour, colour.models.

Documentation

• New features are documented along with examples if relevant.
• The documentation is Sphinx and numpydoc compliant.

[coveralls]

Coverage: 99.784% (-0.003%) from 99.787% when pulling 4d3dc25 on feature/xxhash into 95592e5 on develop.

[tjdcs]

Does this line need #pragma: no cover?

Otherwise I don't see any issues and this looks ready to merge.

[KelSolaar]

Merged, thank you!