Support iterables.

[mbostock]

• [x] pie(data)
• [x] line(data), lineRadial(data)
• [x] area(data), areaRadial(data)
• [x] stack(data)
• [x] stack.keys(keys) constant
• [x] stack.keys(keys) function
• [x] stack.order(order) constant
• [x] stack.order(order) function