Bump firrtl from `4181e92` to `559c667` (#181)

Bumps [firrtl](https://github.com/freechipsproject/firrtl) from `4181e92` to `559c667`.
- [Release notes](https://github.com/freechipsproject/firrtl/releases)
- [Commits](https://github.com/freechipsproject/firrtl/compare/4181e92732f616d4c58409cd59ae782f11967d45...559c66722a858092e92c1b293331b343fc180c03

)

---
updated-dependencies:
- dependency-name: firrtl
  dependency-type: direct:production
...

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