Add RTLIL Backend. (#2331)

* Added RTLIL Backend.

* Add test for Rtlil Backend, fix per-module file emission, scalafmt, and apply bugfixes for inconsistencies found during testing.

* Fix build on scala 2.13

* Add additional equivalence test, make some bugfixes and perf opts to the emitter.

* Final changes as requested by Kevin, code cleanup, add support for formal cells.