V2 tweaks

[jhollinger]

Extracts some minor changes from jh/2.0-renderer that aren't rendering-specific.

• Ensure all field/object/collection names are stored as symbols.
• Splits the V2::Association struct into V2::ObjectField and V2::Collection.
• autoload everything, but manually require files that would have been loaded during render (it's easier for testing).
• Add view_name so the DSL can dynamically refer to the current view's name (like V1's current_view.name).