Convert slice generator to use ruby class file

[cllns]

A number of refactors here too:

• We needed a Module file so I had to create RubyModuleFile. I first copied it from RubyClassFile, but realized I could share almost all of the code. I did this via a super class called RubyFile, so now RubyClassFile and RubyModuleFile are just thin subclasses with their specific behavior. Open to other approaches here. I considered just assuming everything is a class file, then having a module: boolean arg with different behavior, since we rarely ever need to generate modules.
• I removed fully_qualified_parent and partially_qualified_parent (from Refactor RubyClassFile, remove RubyFileWriter #296) and standardized them as (fully qualified) parent_class_name. This involves a bit more code from consumers (manually creating the string of the parent class name) but it seems worth providing a simpler API and implementation.
• I was able to get rid of all ERB from Slice generator. The only remaining thing that makes some sense to have as ERB is the layout file, as that ends up as an ERB file in the end. But it was easy enough to inline with a heredoc (and simpler to not use ERB to generate ERB).