Make unused function pass optional #409

Make a new compiler option, remove_unused_functions. By default it should be set to Mix.env == :prod. Meaning by default it would be true only for production builds.

In compiler, if remove_unused_functions is true, skip the FindUsedFunctions phase.

In the Translate phase, when remove_unused_functions is false, set the used field equal to the definitions field. All other compilation should happen as normal