Clean-slate redesign/simplification of Backend class

[byorgey]

See https://raw.githubusercontent.com/diagrams/diagrams-doc/master/notes/backend-data-flow.jpg (note the diagram is not quite right since the Options should get transformed by the adjust step too).

• Remove constraints on Render type (note we still need it to be able to express the Renderable class)
• Merge doRender and renderData into one function, renderRTree.
• Move renderDia and renderDiaT out of the Backend class. Allowing backends to implement them directly, instead of in terms of renderRTree and adjustDia (the default) was not buying us anything.
• Get rid of NullPrim since it is hard to define without a Monoid constraint on Render, and it turns out we don't need it anyway.