Bump enumn from 0.1.5 to 0.1.8 (#7226)

Bumps [enumn](https://github.com/dtolnay/enumn) from 0.1.5 to 0.1.8.
- [Release notes](https://github.com/dtolnay/enumn/releases)
- [Commits](https://github.com/dtolnay/enumn/compare/0.1.5...0.1.8)

---
updated-dependencies:
- dependency-name: enumn
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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