Referentially transparent logging with slf4j

Don't forget to include an implementation of slf4j-api (such as logback) on your classpath.

libraryDependencies += "com.nequissimus" %% "zio-slf4j" % "0.4.1"

For a simple usage example, see sample.scala.

zio-slf4j's dependency on Scalaz is entirely optional. It will only be used if already available, and not brought in transitively.

If Scalaz is availble on the application's classpath, methods using Show can be used. Otherwise, log messages will need to be of type String.