Bump synstructure from 0.12.1 to 0.12.3
Bump synstructure from 0.12.1 to 0.12.3

Bumps [synstructure](https://github.com/mystor/synstructure) from 0.12.1 to 0.12.3.
- [Release notes](https://github.com/mystor/synstructure/releases)
- [Commits](https://github.com/mystor/synstructure/commits)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
splav/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,splav/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,splav/servo,KiChjang/servo,splav/servo,splav/servo,splav/servo,splav/servo,KiChjang/servo,splav/servo,KiChjang/servo,splav/servo,splav/servo