Accept iterables.

[mbostock]

Fixes #148.

[Fil]

I just tested 5a4e792 on a notebook I was working on today 👍

[mbostock]

Here’s a good example of how iterables support will help:

https://observablehq.com/@mbostock/revenue-by-music-format-1973-2018

Before:

series = d3.stack()
    .keys(Array.from(colors.keys()))
    .value(([, group], name) => group.get(name).value)
    .order(d3.stackOrderReverse)
  (Array.from(d3.rollup(data, ([d]) => d, d => +d.date, d => d.name)))

After:

series = d3.stack()
    .keys(colors.keys())
    .value(([, group], name) => group.get(name).value)
    .order(d3.stackOrderReverse)
  (d3.rollup(data, ([d]) => d, d => +d.date, d => d.name))

Similarly with d3-scale support, before:

color = d3.scaleOrdinal()
    .domain(Array.from(colors.keys()))
    .range(Array.from(colors.values()))

After:

color = d3.scaleOrdinal()
    .domain(colors.keys())
    .range(colors.values())