Yet another slicer. This one picks up the idea from sfc/slicer.py but
implements it without using FreeCAD. The FreeCAD-based solution turned
out to produce weird artefacts for some reason, e.g., open paths that
were then falsely closed and yielded bogus toolpaths.

This slicer uses very strict conditions for horizontality: all the
points of a horizontal facet must have identical Z coordinates. It
also implicitly ensures that all paths are closed.