R1:612b29fff40a

Implement basic one-up and test renderers

Summary:
This is a half-step toward one-up and test renderers. This is mostly a structural change, and has no user-facing impact. It splits the rendering hierarchy like this:

- Renderer (more methods are abstract than before)
- HTML Renderer (most HTML stuff has moved down from the base to here)
- HTML 1-up (placeholder only -- not yet a functional implementation)
- HTML 2-up (minimal changes, uses mostly old code)
- Test Renderer (unit…