Rename `arcAC` to `arc'` following the [sunroof compiler's](http://hackage.haskell.org/package/sunroof-compiler-0.2/docs/Language-Sunroof-JS-Canvas.html) lead.
Use the attribute framework from `Core` to set attributes on the
canvas context rather than exporting setter functions.

Thus, we now also support getting the value of attributes. However,
the two examples are less nice with this change. Maybe the examples
should change.