Convert Part generator to use RubyFileWriter and Generate::Command

[cllns]

Builds on #288

After this is merged, I want to refactor some of this. I don't love the names relative_parent_class (gets the app/slice namespace prepended) and absolute_parent_class. Considering using qualified/unqualified but that's not super clear either. Any ideas? parent_class and parent_class_within_namespace?

In that PR, I'll also go through and change all references to RubyFileWriter to RubyFileClass, since that's the API we want to use going forward.