Allow d3.map to be used with d3.nest’s map.

You can now pass a map type to d3.nest’s map function. For example, if you pass
d3.map as the second argument to nest.map(array, type), the returned value will
be an instance of d3.map rather than a bar object. In addition to offering
better protection against built-in property names, d3.map offers more convenient
iteration methods.