Bump proc-macro2 from 1.0.49 to 1.0.51

Bumps [proc-macro2](https://github.com/dtolnay/proc-macro2) from 1.0.49 to 1.0.51.
- [Release notes](https://github.com/dtolnay/proc-macro2/releases)
- [Commits](https://github.com/dtolnay/proc-macro2/compare/1.0.49...1.0.51

)

---
updated-dependencies:
- dependency-name: proc-macro2
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>