Bump syn from 1.0.63 to 1.0.64 (#143)

Bumps [syn](https://github.com/dtolnay/syn) from 1.0.63 to 1.0.64.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](dtolnay/syn@1.0.63...1.0.64)

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