Cleanup pass.
Cleanup pass.

- Created a +by-build-dag door which encapsulated the bijug logic.
- Did some reorganization of +reduce.
- Started trying to comment better.

ngzax/urbit,jfranklin9000/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,urbit/urbit,joemfb/arvo,urbit/urbit,ngzax/urbit,jfranklin9000/urbit,jfranklin9000/urbit,joemfb/arvo,belisarius222/arvo,jfranklin9000/urbit,ngzax/urbit,urbit/urbit,jfranklin9000/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,belisarius222/arvo,ngzax/urbit,jfranklin9000/urbit,jfranklin9000/urbit