Bump syn from 0.15.30 to 0.15.31
Bump syn from 0.15.30 to 0.15.31

Bumps [syn](https://github.com/dtolnay/syn) from 0.15.30 to 0.15.31.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](https://github.com/dtolnay/syn/compare/0.15.30...0.15.31)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
alex/otp-cop