ordering is not correct for ordinal (string) keys

[gordonwoodhull]

We removed crossfilter.quicksort in #1560 but the way we implemented it using Array.sort() is only compatible with numbers, not with strings.

I was looking at how hard it is to implement ordinal bar transitions for a question on SO. It's not hard, but the ordinal transition tests that modify the ordering were not working.

In order to work with both strings and numbers,

Array.from(data).sort((a, b) => { return _ordering(a) - _ordering(b) })

should be

Array.from(data).sort((a, b) => d3.ascending(this._ordering(a), this._ordering(b)))

so that it does not return NaNs (which effectively disable ordering).