Top sheet has holes
Use top_plate() rather than one-off
Progress on another y carriage
New x carriage, work on z axis

World's messiest commit.
sketching out ideas for Z axis

The centrally supported z axis is more scalable (a 12x12 or 24x24
printer would require quite the z rod and vertical bracing).  We'll see
if the linear bearings are stable enough.  Cheap bearings might rock more
than desired.  Maybe use three bearings or use lm8uu for Z even on the
m6-size printer?
Clamps for x rods