Bump monostate from 0.1.6 to 0.1.8

Bumps [monostate](https://github.com/dtolnay/monostate) from 0.1.6 to 0.1.8.
- [Release notes](https://github.com/dtolnay/monostate/releases)
- [Commits](dtolnay/monostate@0.1.6...0.1.8)

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

Signed-off-by: dependabot[bot] <support@github.com>