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.
this seems like a really inefficient pattern. basically it reduces to
```
for x in alist:
    for y in alist:
        do expensive computation
        if x === y:
            use the computation
```

can't this be refactored into a single loop?