Bump proc-macro2 from 1.0.8 to 1.0.9 (#908)

Bumps [proc-macro2](https://github.com/alexcrichton/proc-macro2) from 1.0.8 to 1.0.9.
- [Release notes](https://github.com/alexcrichton/proc-macro2/releases)
- [Commits](dtolnay/proc-macro2@1.0.8...1.0.9)

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