inline facets and axes

[mbostock]

A few internal improvements derived from the mbostock/layout branch:

• Don’t wrap all marks in a Facet mark when faceting; handle faceting directly.
• Don’t try to treat AxisX and AxisY as marks; render them directly.
• Override mark.filter instead of passing defaults.filter to the constructor.
• Rename a few local variables.

[Fil]

This is beautiful!