* Bump breeze and spire versions

* Bump spire to 0.18.0
* Bump breeze to 2.1.0
* Override method 'reverse' in SignAlgebra due to both Order[A]
and CMonoid[A] now having 'reverse' methods each.
* Remove parameterless non-Unit method definitions and invocations

* Bump Chisel version to 3.6

* Bump dsptools version to 1.6
* Bump chiseltest version to 0.6
* Bump scalatest version to 3.2.15
* Bump fixedpoint to version supporting Chisel 3.6.0
* Apply fixedpoint.shadow.Mux to more places
* Add '-Xfatal-warnings' compiler option
* Remove Saturate as it uses firrtl transformations
* Add branch 1.6.x to CI workflow

* Update scalafmt settings

* Add sclaafmt sbt plugin
* Reformat source files according to scalafmt rules
* Add scalafmt step to CI

* Changed license from BSD to Apache-2.0 in README.md

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Co-authored-by: Aleksandar Kondić <konda@uni.kg.ac.rs>