Converting d3.phylogram.js to a Vega transform

A data transform, that is. Actually two of them (this may change): one
for the main layout and projection, and another for our special link
paths. Since the path geometry is dependent on the phylogram
orientation, I'll probably resort to pre-baking paths and attaching them
directly to links, to be applied later in the rendering phase.
Finally, a proper phylogram display.

This uses the modified phylogram code as a Vega data transform. Still
working out the cleanest way to meet our special requirements during
each stage of the Vega pipeline (source > transforms > rendering).