Scale doesn't know about data, renderer does

Before, scales knew what their data was and would compute an extent from that whenever it updated, with updates flowing like `renderer <- scale <- data`. However, sometimes the extent is better known by the renderer, for example, a bar chart knows that it must include enough space for the last bar.

This branch rearranges things so that scales don't know about their data, they only have  a renderer to extent mapping. The new flow of control in `scale <-> renderer <- data`, where a data update causes the renderer to compute a new extent, send that new extent to the scale, which potentially changes its domain an signals back.
it looks like if the renderer changes what scale it is pointing to, the renderer's extent will never get detached from the old scale. that is not good.